Merrymen are off-screen characters. They are dressed in green pyjamas. They take from the rich and give to the poor. There is a chance that they will give/take stuff from your shelter. the less items in the shelter the higher chance of receiving things and the more stuff in your shelter the more likely they are to take things.
A group of people dressed in green pyjamas knocking on our hatch is not something we anticipated. Yet this is exactly what happened. Furthermore, they insisted they are 'Merrymen' and wish to take from the rich and give to the poor. We're pretty poor, so maybe we could make some new friends? Should we let them in?
If they say you are rich and you let them in:
Those 'Merrymen' were not merry at all. They told us that we were rich, and that we should share with the other 99% of the wasteland. They grabbed a few things and left. We couldn't do anything to stop them - their green clothes were too damn hypnotizing!