Well, I know if you have the "Kill enemy players" quest that group kills count toward the overall number. Not sure about Xp though.
My Character:
Priso: VR12 High Elf Sorcerer. Guildmaster of the Blood Knights of Sithis.
My Guild:
The Blood Knights of Sithis is a 170+ man Vamp PVP guild that does it's PVPing in Wabbajack. PM me in-game or on the forums for an invite! All vamps are welcome!