Alright friends, here's the deal:
Three rares for ally and three rares for hordies have a chance to drop an invitation. I'm ally, so the three that I farmed are:
Kar Warmkaer (68321)
Ubunti the Shade (68320)
I got it on the third kill. When you hit rank 7 your reward is ONE pass to give out. So please don't harass everyone in the arena area for an invitation.
Good luck with your farming!
3rd kill? I got one to drop on like 12th kill of 16 before me and my friend gave up because other people started to catch on @_@ He won and he doesnt even know what brawlers guild is>.>
But thanks for that info about rank 7, time to go force him to get to that part XD