Blizzard revealed their plan for the Cross-server Battlegrounds in Patch 1.12 – All US and Oceania realms will be divided into 8 Battlegroups. Each Battlegroup consists of 16-20 Realms. Check out which Battlegroup<
your realm belongs to in advance.
Eyonix: “The next content patch will allow you to join players from other realms in the battlegrounds. To designate which realms will be playing with each other, all realms will be organized into battlegroups—each battlegroup is a collection of realms that share their battlegrounds. You can now check out the full battlegroup list to find out which battlegroup you’re in and which realms you’ll be competing with once the next content patch is released.”