I would like to know how the group formation works in BG. Do I only play with players linked to my alliance or is it a mix of players from the 3 alliances?
Internally, each player is 'assigned' to one of the three alliances (which can be seen if a player is wearing the Alliance costume or a DK uses their banner), but for all intents and purposes the alliance does not matter. You're given your team and you may see others on your team who normally fight for another leader.
If you do a group queue with friends, if you go into cyro or ic while waiting, you will need to be the same faction through while in queue
PC NA @gariondavey, BG, IC & Cyrodiil Focused Since October 2017 Stamplar (main), Magplar, Magsorc, Stamsorc, StamDK, MagDK, Stamblade, Magblade, Magden, Stamden