I don't belong to any social groups nor an I a member of admin. So I'm not the best member to be replying to your post. However, since you have yet to receive a reply I thought I would at least try to get you started.

If you go to the index page & scroll down to the list of social groups, you can click on the particular social group you want to join this will then take you to that social group's page where the various threads that have been posted are listed. At the top of the page is an explanation as to how to join the social group. (It starts out: "You're browsing a Social Group forum.") Follow the directions given in that paragraph and it will get you where you need to be to join the group.

I might just mention, though, (perhaps you're already aware of this) few of the social groups seem to be at all active anymore. There might be 4 or 5 I can think of off-hand that are active. But the rest seem, at least, be pretty-much dead. Still... perhaps you can revive the ones you're interested in? Good luck!