Lets say there is an object(game) that has an array with objects(players) and if the player array has a certain value(_id) that equals the current value I want to hide a button(join).
<button ng-hide="playerJoined">Join game</button>
You can ng-hide on a
$scope.inGame() function that checks
$scope.game.players for the given ID. Like so: http://jsfiddle.net/mxhy0sdd/
P.S. Don't use $rootScope to share data between controllers :P I fixed this as well