Since I do a lot of online roleplaying it'd be helpful to have my Notify list grouped into folders so that I can see which members of my "raiding party" are online at a glance. But I don't see a way to do that. Is it possible with mIRC?

This doesn't do what you want, but you could always put a note in the notify list of RAID so at a quick glance you can see who's who. I have mine setup to show me services and ircopers using the NOTE option, but you can have it say anything you want it to.