A.H.
If he's not hiding anything about these friendships, I see nothing wrong with them. If it were ONLY the females that he was looking up, I might be concerned, but it sounds like he's just wanting to reconnect with some old friends. What's wrong with that?
Why do you feel that getting together with old friends in a group is inappropriate? Why not go along with him to meet them? Were these friends really bad influences or something?
My husband and I get together with friends fairly often, sometimes with the kids, sometimes without. But we are very open about who we're with and what we're doing--and we always invite each other along. If I want to go out to lunch with a guy friend of ours, I talk to my husband first. If he isn't going with us, I let him know where we'll be (always a very public place, not "romantic"--I don't want to give ANYONE the wrong impression) and what time I'll be home, and he knows he can always "drop in" on our visit if he feels like it. It's also vice versa (although I admittedly have more male friends than he has female friends). We trust each other, and we don't do things that we wouldn't want the other to do.
I guess I don't know why you're so worried about it. Has your husband given you a reason not to trust him (cheated in the past, etc.)?
Now, if the time he is spending on the computer is taking away from meeting his family's needs--regardless of whether he is chatting with friends, surfing, playing games, or whatever--then it IS a problem that does need to be addressed. But if he's meeting the family's needs, and he's using his own "free time," and he's just chatting (not flirting)... then, to be honest, it sounds like you're just insecure.
Maybe I'm wrong and there is more to it, but that is just what I got from your post.
Good luck!
--A.