It was pointed out to me that this would not work this way. Instead, I am now iterating over all my creeps to find the roles I want:

function GetCreepsByRole(role){
  var CreepList = [];
    for (var creepname in Game.creeps){
      if (Game.creeps[creepname].memory.role == role){
      CreepList.push(Game.creeps[creepname]);
    }
  }
  return CreepList
}

Hello everybody!
I am new to the game and haven't done much JavaScript. But I am enjoying it a lot and am using the forum and StackExchange to slowly work my way forward. Today I encountered one problem that I think was worth asking abut here:

Game.spawns.Spawn1.room.find(FIND_CREEPS, {filter: function(object) {return object.memory.role == 'Harvester'}});
//Returns all Harvesters in the room, as expected. Individual elements of the result can be passed to creeps as commands

or

_(Game.creeps).filter({ memory: { role: 'Harvester' }});
//Returns all my Harvesters in the world, as expected. But individual elements are not recognized by commands to other creeps.