gistfile1.txt

@RemoteAction
global static Boolean addUserToSession(Id userId, Id sessionId){
  if(space=='avail'){ return true; }
  else{ return false;}
}

From javascript:
var userId = '{!JSENCODE($UserInfo.Id)}';
var sessionId = '{!JSENCODE(controllerHasSessionId)}';
myControllerName.addUserToSession(userId, sessionId,function(result){ 
  if(result == true){
    popGoodModal();
  }else{
    popBadModal();
  }
});