Blizzard is testing a new Real ID Party feature in World of Warcraft.
It lets you invite Battle.net friends who use Real ID to your five-person WOW dungeon group - regardless of the realm/server they play on. You can all play together at the click of a button. But you must all belong to the same faction, either Horde or Alliance.
Real ID Party only works with five-person normal or Heroic dungeon groups. Friends on other servers must use the Real ID Battle.net feature for Real ID Party to work. You can only invite your Real ID friends, not friends of friends. Groups can contain Real ID friends and normal players from your server.
Blizzard hasn't decided how long Real ID Party testing will last.
A Real ID Party FAQ on the World of Warcraft website explains the finer points.
Real ID is an opt-in feature on Battle.net that lets you display and share your real identity with other users. This allows you to keep track of your friends across games even chat to them while they're playing StarCraft II and you're playing World of Warcraft, for example.
A Real ID FAQ offers a more in-depth explanation.