What I do in-between BGs and TB, I just ruin the life of anyone from horde who has decided they want to try and do TB dailies on my watch.

As for getting into the same group in a TB battle, just ask the leader for assist and move your guys around in the raid. Not 100% sure if this still works, you used to be able to kick people from the raid and invite your own toons if they were in a seperate raid group, but that got changed.