This Yahoo! Group was first designed for ex-members of the old Prodigy Archaeology Bulletin Board. Prodigy was commonly abbreviated "*P" or "P*" hence ex-P* or simply XP. Okay, admittedly it's not a very clever name, but it's short and has a nice sound to it.
The name has nothing to do with the Microsoft operating system, Windows® XP, the name of which was announced some time after this group was formed. I've half expected a note from Microsoft's legal department (these guys are as possessive as Disney), but perhaps the group is too small to be noticed.