This is the code I use to play screeps.
// create a creep
Game.spawns['Spawn1'].spawnCreep( [WORK, CARRY, MOVE], 'Harvester1' );
Game.spawns['Spawn1'].spawnCreep( [WORK, CARRY, MOVE], 'Upgrader1' );
// assign the role in memory
Game.creeps['Harvester1'].memory.role = 'harvester';
Game.creeps['Upgrader1'].memory.role = 'upgrader';