It is critical to understand, the Medieval Underground was not a monolithic body. It manifested itself in many different ways and in diverse locales.
A sort of proto-feudalism was already developing in the waning years of the western Roman Empire. There are reasons for this, one large factor being security. The manor-centered life provided protection. The peasant lived in a clustered village with the fields surrounding the community. Labour was combined for efficiency and as the Roman system expanded the village was intimately tied in with the rhythms of Sacral life. Village priests were often poor and uneducated and completely dependent on the local lord. The piety of the Lord would greatly affect the depth and grip of Roman Christianity in his holdings. If the Lord was impious, he might hire a worthless, drunken, whoring priest who would care little for the 'flock' and thus the people would have a good deal of freedom. If there were Dissenters present, by being quiet and law-abiding they probably wouldn't experience any trouble from either the Lord or the Priest. In fact there are cases of Roman Catholic Lords more than happy to have Dissenters as subjects because of their peaceful law abiding industry.
Now in other circumstances, a Lord might be quite pious and employ a zealous priest who would demand conformity, regular church attendance, and encourage the other villagers to report suspicious behavior. This would be a hostile environment for a Dissenter. At that point, they had a choice to make. You could often get away with attending church just a few times a year. They could stay in the village near their group and their family and play Nicodemite on occasion…or they could flee which was both costly and dangerous. If the Lord pursued them, the punishment could be severe.
But often times they were not pursued. And they could go somewhere else…undoubtedly the networks reported the 'good' places…and apply to another Lord for a plot of land. That was one option.
Feudalism was not the same everywhere and in some places, men could attain the status of Yeoman or landholder giving them a bit more freedom in their daily life.
The Normans introduced feudalism in England. Prior to the conquest the Saxons and Danes had followed the Germanic custom of private land ownership. There are no records of heresy in Saxon times…some would say because it wasn't present. We too can argue from silence and say, culturally it wasn't an issue. Sacralism had not yet taken hold of the societal structure. The society was still somewhat pluralistic, pagan, Roman Christian, and perhaps Dissenter living side by side. The Saxon church and culture was less likely to care if people attended Mass or not. The Chaos of the Dark Ages was destabilizing but would allow for great freedom for those questioning the growing Sacral monolith. There were also many more out of the way places then…and areas that were virtually autonomous. Even with feudalism there were many places in the mountains, forests, and dales where people were not touched in their daily lives by the Sacral system. Shepherds in the crags of Yorkshire and farmers in the Peak District as well as Foresters in Wiltshire and Hampshire would have had more freedom than a peasant in Sussex. There are also long traditions in Scotland and Wales, lands long free from the Roman yoke. Unfortunately many of these accounts are entangled with nationalist sympathies and agendas which decreases their value, but doesn't erase the fact that there was something there. The Welsh church has a long but nebulous tradition of rejecting Roman rule. They have a tradition of monasticism quite different from Benedictine order. Their monasteries were something more like communes, whole families with married clergy living together and practicing a somewhat unclear type of Christianity.
Another option was to flee to a town. During the Dark Ages, approximately AD400-900, town life was in decline. Trade had mortified and the countryside was dangerous. But as we enter the High Middle Ages, towns were flourishing and they were a great haven for the 'heretic'. Town life meant being free from vassalage, and allowed for a certain degree of anonymity. It was much easier to stay quietly in your house on Sunday morning in a town and not be noticed than it was to skip church which you lived in a village with less than a hundred people. In the village everything you did would be noticed. Anyone who lives in a small town can attest to this. In the city, one can escape notice and it is unlikely that anyone but the most zealous priest is going to make the rounds and inquire at every house.
Certain cities like Venice and the later Hansa towns were even better, because of their cosmopolitan nature. The citizens were used to seeing all types of folks, including Jews, Muslims, Orthodox…all engaged in trade. Many of the permanent foreign residents had their own quarters in the city, but the overall picture presented to us indicates a bustling chaos in which a Waldensian could remain unnoticed. Venice in particular was said at different times to contain thousands of Dissenters. I thought of them often as I wandered those narrow alleys and canals. It paints quite a picture. These were also excellent locations to hide brethren on the run, it would provide a transportation nexus, and could serve as a central location for copying books and distributing them.
The profession of Cloth merchant, and Weaver became popular among the dissenters. This allowed excuse to travel, networking with other brethren abroad and no one looked twice at hoards of people coming and going from the homes of such merchants.
It was the perfect cover. Some of the guilds became dominated by Dissenters and were often under suspicion. Also a traveling merchant could hide people and books in their wagons full of goods. Some of these Dissenters did very well and became rather affluent and used their money to support and sponsor others. These Waldensians also seemed free from much of legalistic piety you find with some of the other groups like the Unitas Fratrum.
Women are sometimes mentioned as preachers among the Waldensians. I don't doubt it is true but it depends on how you define preaching. If you mean women going into homes and sharing the gospel…well, I have no problem with that kind of preaching. If you mean women helping to distribute the Sacramental elements….that is not necessarily a problem. If you mean women functioning as elders …that could be a bit more troubling. But I have yet to find any convincing evidence of this. One cannot assume a church structure akin to what we have today. There were instances in some places of quasi-monastic communities and some especially for women…but it is unclear as to what exactly was happening there. As I've said, they weren't monolithic. Not everything they did was right and some groups or cells would have been more sound and orthodox than others. I'm not looking for Institutional Form Unity. This seems to be a hang up for a lot of people when looking at these groups.
Another option was to take to the forest or the wilderness. This also was common though for obvious reasons it's hard to get a sense of the numbers. As mentioned elsewhere some testified to the presence of thousands, even tens of thousands living in the Forests of central Europe. These were positions outside the mainstream of society, under no protection of law, but safe from those who would persecute them. There are stories of whole forest communities coming down to us from the middle ages. We cannot say with certainty that some of these were Dissenters, but it is almost certain. If there were indeed thousands in the forest, than they wouldn't have lived an existence of sleeping in tree hollows and caves. They would have constructed primitive settlements and it is likely due to the numbers caught on the road….these people traveled and moved about and had connections in different places.
We know many Waldensians were traveling merchants, some were even troubadours.
As far as cave and forest meetings…this would be people from feudal villages escaping in the night to meet…perhaps sometimes with the local forest group. They couldn't meet in the village…they had to sneak out. In the towns, houses would work.
The most famous Waldensian group lived in several valleys in the Cottian Alps, originally on both the French and Italian sides. The French valleys later declined under persecution. This was a frontier, a marchland where jurisdictions were hazy. Lords were unlikely to prosecute and hunt heretics despite the urgings of a Bishop or Inquisitor. The Alpine Waldensians did suffer extensively but there were also long periods of peace. As their Barbes or Pastors left the valleys carrying copies of the Scriptures in the vernacular they faced danger. If caught, they would be imprisoned, tortured, and often burned. But there was an evangelical zeal about these folks. They worked to spread the message.
The Lollards faced a similar situation…some lived in the Fens of East Anglia, some in the Forests of Wessex, some on the Welsh March where there were traditions of dissent hearkening back to the Dark Ages and the presence of the nebulous Culdees.
This was wild hill and forest country full of places to hide.
In many ways the town option of the High Middle Ages was probably the best and safest, but it wasn't available to everyone. The towns were a difficult place to be if you didn't have a skill or trade and many had to live in the country. If that was the case your best hope was to find a Lord who was favourable or indifferent, or their only other option was to become out-law…and live outside the society which many did.