To talk of course. They either have something good to say about someone in the group they are with it is most likely that they like that person. If they have something bad to say it is either they are just going to complain or they are going out with one on the guys with you. They other girl(s) will make matters worse if it is about a breakup. So heres a tip. GO WITH ONLY ONE GIRL!!! haha this forum had no point to it. If you have any other ideas. please post and help those to oblivious to see what is happening around them. XD
I'm not sure how many of the replies here are from girls, so I'll just add this as a girl's perspective:
It's to talk by themselves so the other people they're with can't hear. If you're dating one of them, it's probably to talk about you. That's pretty much it.
Most guys are not aware that all girls suffer from public urophobia. I am one of the few guys who has this valuable piece of information. Don't ask me how I got it or why I want to know it.
So, as a matter of fact, one can not pee alone if a girl they be.