It's still not clear exactly how Alliance Battles - the new 12v12 format in Factions - will work in terms of party creation. But Gaile did give a couple more details
12v12 is not random, in that you form a party of 4 and work with 2 other 4-party teams.
I don't think it's constrained by alliance, but that may become part of the mechanics.
It's my understanding that you will not be able to do 12v12 completely randomly.
That it will require parties of 4.
Sounds like they haven't completely made their minds up. Still, requiring parties of 4 should substantially increase the level of clue in the format, which would be nice. I'm still a bit worried about the "exploit" where 3 guild teams hit "join" at the same time to all be on the same team. If that actually works then it will be required to be competitive, which would suck. Getting 8 people together for GvG is a pain as is - getting 12 would be even worse.