Are you joining as a group or joining using AV preform enabler?

The leader of the largest group to join a BG will become the BG leader. So if it's you, go to the raid panel on you social tab or w/e, and click and drag the names to the groups you want. if you are not the leader, just ask whoever is (nicely) to do it for you.

PS: AV preform enabler is just like joining solo and coincedentally ending up in the same BG, it won't get you the leader except on rare occasions.