If it is a group, you have to go to (1) your profile, (2) the social groups area, (3) request to join that particular group - which may or may not allow you to join automatically, and then (4) you can start replying to those groups.

I tried to create some groups, but I'm not sure how to manage them. Forgive me, Sarahsweets, if you tried to post in one of my groups. I suppose I could open it up to the public, but then I think the public (not just members) could view it, which I thought wouldn't be cool to others. So I left the group as members only, I think. I'm not sure what I did with that.

I don't know how to invite others to a group either.

I'm sorry if I created the confusion, Sarahsweets.