The context for Earthen becoming playable is Magni cleanses and fixes the machine that allows for the awakening of new Earthen. All of the story quests required to unlock Earthen are in alpha already, only thing we are missing is the player Earthen intro.
But no major group joins one side. During the quest, the Bronzebeard family and you the player basically re-unify Earthen society so all the groups are working together and happy with each other now. Adelgon, the leader of the Unbound retakes her former position as Dornogal's Stoneward and takes interim command of Dornogal's military after the Stormward Baelgrim's death. The rest of the Unboudn rejoin the Oathsworn after the Dorn quests. After the RInging Deeps, the Machien Speakers are saved by the Player and the Bronzebeard crew and also rejoin the others now that the coreway is cleared and Brinthe is named High Speaker of the machine speakers after Moira nominated her.
And of course Magni giving up his Azeroth powers to cleanse and repair the Hall of Awakening allowing new Earthen to awaken (this is the last story quest requirement needed to unlock player Earthen)
My guess is newly created Earthen will start her in the Halls of Awakening since that area is its own closed off instanced bit