I like the group idea and would like to add a little to it.
Custom groups. Let people define their own groups to send to, like mailing lists. Set the ability to do this per usergroup. This way, I could for instance let members setup their own groups so they could flag a custom group of people based on THEIR criteria, not mine (as in per xF usergroups).
I think the xF usergroups is a great idea as well, I just think custom ones would rock too. =)