I think there needs to be a chat messenger in tremulous where you can add friends, see what game they are in, message them in that game, and so on.
This would be very useful.
Download xfire, rename trem.exe to quake3.exe and point xfire to it.
I have xfire, but will making it think Tremulous is Quake 3 allow you to join games from the messenger?
Yup. There was an older thread about it, try searching for a topic called "xfire".