Lets Look at Cross Faction Guilds in world of Warcraft

From the Blizzard Post:
Some things to remember with cross-faction guilds:

Guild affiliation with the Alliance or Horde will depend on the faction of the Guild Leader, and guild achievements and vendors will still reflect the guild's primary faction.
Guild members of the opposite faction will benefit from unlocked shared perks but cannot contribute to specific achievements. As an example, Alliance members of a Horde guild could not contribute progress towards the "Alliance Slayer" guild achievement.
Guild repairs and social conveniences, like shared chat, will be available to opposite-faction guild members.
Players must be Battle.net friends or part of the same Battle.net community to invite or receive an invitation to an opposite-faction guild on the same realm.
Opposite-faction guildmates can only play communally inside instances; they'll still be considered Unfriendly to each other (or Hostile in War Mode) in the outdoor world.

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