How about access only for premium members with watching open for all and participating on invite only.
It works a bit like this:
You're a premium AChat member.
You open the list of “public” rooms, choose one and enter.
While lounging on a couch you see a friend (or perhaps two friends) having a good time, there are other couples working up a sweat as well.
You send your friend(s) a request to join them. Both agree and you form a threesome with them.
The voyeurs on the couches/chairs/whatever would be dressed and can perhaps cheer, whistle or applaud and rub themselves over or inside their clothes.
From here the step to a social club where people can go to dance, consume beverages and do drunken things like throwing up, falling over etc. and chat in general (and take someone to the toilets/broom-closet/alley in the back/abandoned office for some quick relief) is relatively small.