how groups work. Can anyone give me a reference to use if there is such a thread?
If you're not talking about teams. Join the group that interests you most, or if it's moderated, you need permission from the creator of the group in order to join. "Invite only" groups, for those, you need to be sent an invite from the creator in order to get in.
They're basically just mini forums that can be created for a specific topic or whatever the case may be. As Orion stated, there are some groups with moderation on them where you can only be invited to join. Each group has its own individual "mini-forum" for discussions and posts.
Though apart, forever connected.
Above sig courtesy of Kayarine, sigs in spoilers courtesy of Kurayami and Waffles. Thanks to Mizu for the sketch of Airal (my avatar)!