Interesting question Robert. As in real life, your friends here should be anyone you decide to be friends with. I have not heard of a method of deciding friendships. You may want to only be friends with people within the groups you join. Or you may decide not to be friends with anyone, only interacting via the groups. My last 2 blog entries may help you make up your mind...I guess the bottom line is..it is totally up to you.
I hope this answers your question.