Worker-task assignments represent one of the critical issues in crowdsourcing, as they affect the quality of task results. This study addresses the problem of forming worker groups assigned to the same task in a task stream that requires more than one worker. We introduce a worker-group queue model that covers practical and common scenarios for task-stream crowdsourcing, and compare three strategies in terms of the skill balance among worker groups, the quality of the final outputs, the number of worker re-assignments of workers, and psychological stress felt by workers. We found that one of the compared strategies that employs multiple worker queues yields good results based on these measures.