Are you talking about Ravelry groups?
I'm in a bunch and I'll probably only add to that. I join them for two reasons:
1.) It's a local group with meet-ups and the board provide location info, inside dish on LYS sales, I might be able to meet actual IRL friends from it, etc.
I keep a casual eye on the boards for these groups to catch the info.
2.) I peruse somebody's profile, see a really silly group or something I'm a fan of and go "Bwah!" and immediately join said group.
Once I've done that, generally speaking, that's the only attention paid to it. I don't share projects, don't keep track of the boards, etc. I guess it's sort of like collecting online tchochkes.
Embarrassingly enough, the majority of my groups fall into category 2.
ETA--I do pretty much the same thing with my Facebook groups addiction...