Tyren on the Blizzards forums posted this about the upcoming battlegrounds:

The following is a step by step process regarding the battlegrounds queue system. The battlegrounds are instanced, and have a limit as to how many players can enter, though this number is unique for each battleground. Thus, we have implemented a system to allow players to play in whichever battleground they choose, or join their friends in a specific battleground instance.

Click here for the full details.