In Patch 3.3.0 cross realm instances were introduced, and they are connected in the same groups as the battlegrounds.

With the introduction of World of Warcraft: Cataclysm and Rated battlegrounds, battlegroups no longer apply to battlegrounds.[1][2] Players from different battlegroups can PvP against each other in both random and rated battlegrounds if they are in the same region (US, EU, etc.).