You can join, just please no same faction killing except for:
Laughing Coffin
Ridiculously Good Looking
Sea Ants
Ghosts of War
(Or if they attack first)
you can steal from same faction, but avoid stealing from:
Our guild
Blitz
Cult of Dinner
You can kill all of the enemy faction you want  |
Wait they have an enemy faction system, but not a guild alliance system? Time to send emails.