Of course, my initial thought was to split the FAQ up and make a new one for communities and member of. I still wouldn't be against that, but I kind of figured this would be the easiest route and would probably work just fine.
Rah wants a new FAQ for it if we're adding info, so I pulled comms stuff out and came up with this.
"How do I hide communities that I am a member of?"
If you wish to be a member of certain communities without them showing in your "Member of" list, you have the option of banning those particular communities or hiding the entire list. Neither of these options make it possible to remove a community from the list of communities to which you have posting access nor hide any communities that you may be watching on your friends list. Because you have complete control over communities that you have posting access in and the accounts you list as friends, LiveJournal does not allow you to hide these lists.
Banning to hide only certain communities from the "Member of" list can be done in the same way banning a user from commenting would be done (http://www.livejournal.com/support/faqbrowse.bml?faqid=20). Even if you have banned a community, you will still be able to post in it.
Hiding the "Member of" list entirely is done by hiding your "Friend of" list (http://www.livejournal.com/support/faqbrowse.bml?faqid=185).
As with hiding portions or all of your "Friend of" list, hiding portions or all of your "Member of" list does not make the information entirely inaccessible. In addition to it being accessible with the Directory (http://www.livejournal.com/support/faqbrowse.bml?faqid=123), you will also be listed in the individual "Members" list for every community that you join. The only way to remove a community entirely from your User Info page and remove yourself from its Community Info page is to leave it (http://www.livejournal.com/support/faqbrowse.bml?faqid=82) and make sure that you are not watching it (http://www.livejournal.com/support/faqbrowse.bml?faqid=61).
If you would like to be an active member in a community but do not want others to know that you are, you may want to create a second account to do so (http://www.livejournal.com/support/faqbrowse.bml?faqid=104).
185 would need editing to pull that info out, of course, but I'm not sure how much should come out.