Cross-faction
Cross-faction is a term used for features that allow players from the opposite factions (Alliance and Horde) to group up.
There are several features that are now cross-faction, however with some limits, which include: Communities, Guilds, Groups, and Instances.
Communities and instances
Patch 9.2.5 made the first step in allowing players of the other faction to play content together. These changes include grouping up with friends of the opposite faction directly if they have a BattleTag or Real ID friendship or part of a cross-faction Community. And yea, players can create and join the same communities together as long as they are marked as "cross-faction".
Premade Groups now have an option to allow those of the opposite faction to sign up. The other activities through the Group Finder like Heroic dungeons, Skirmishes, and Random Battleground remain same-faction.
Most instances were made available to be conquered by both factions together while some remain restricted to the same-faction only:
- Trial of the Champion
- Trial of the Crusader
- Vault of Archavon
- Icecrown Citadel
- Baradin Hold
- Siege of Boralus
- Battle of Dazar'alor
- Darkmaul Citadel (Exile's Reach dungeon)
Guilds
Patch 10.1.0 took things a bit further with cross-faction Guilds.
Some things to note:
- Guild affiliation with the Alliance or Horde depends on the faction of the guild leader and guild achievements and vendors will reflect the guild's primary faction.
- Guild members of the opposite faction will benefit from unlocked shared perks but cannot contribute to specific achievements. For example, the
[Alliance Slayer] achievement cannot be contributed to by Alliance members. - Players must be friends or be part of the same community to receive an invitation or invite players from the opposite faction.
Patch changes
Patch 10.1.0 (2023-05-02): Guilds are now cross-faction. 
Patch 9.2.5 (2022-05-31): Communities, Grouping up with friends or via Premade Groups, and most instances are now cross-faction.
