In practice what happens with "closed" membership groups is usually that there's a single administrator or a small group of administrators who have the authority to add people,,, and they don't have the energy to actually check whether people are serious about the group or to encourage positive participation from them, so they just add pretty much anyone who requests to join,,, and then when that doesn't work any better than an open forum because it's not really a barrier to entry, they impulsively ban whoever seems to them to be a problem,,,,, which is often actually the VICTIM of a troll playing the game of staying within the bright lines of the rules while also trying to provoke other people as much as possible. :/ So that's hardly better than open fora even though it's a bunch of work and stress and drama. What I recommend is closed groups where EVERY member of the group has to approve a request of someone new to join. That makes basic sense in terms of privacy, because it can easily violate expectations of privacy if you're posting things that are supposedly private because they're restricted to members but then the membership can be changed unexpectedly without your permission. I think it would encourage groups that have more of a real sense of group identity, where everyone feels that they've been actively welcomed to the group and taken on some responsibility to the other participants rather than just assuming that some administrator absentmindedly bothered to click "accept" on a list of people. Obviously there's the disadvantage that participants who drop out entirely or participate very slowly can slow the growth of groups, but I think it's worth the cost of making new groups with the active participants. The way I'd write it would be to have every aspect of the group administration be by consensus, so that it also takes agreement of all members to change the title or description of the group or whatever other powers would normally be assigned to administrators. In my experience group administration by an individual or small subgroup only works properly if you COMPENSATE them somehow-- if the only thing you get in exchange for being a group admin is the power to arbitrarily ban people, it's no surprise that people take whatever satisfaction they can out of that power and/or do their job as lazily as possible, what else should we expect when asking someone to do something difficult for free.