drupal_queues for drupal 6
There's no concept of a "sub-queue", it's just queues, items in the queue, and workers of a queue (on items). Without knowing more about your use case I'd say you should may want to have a queue per account. Or create a queue item containing the account its for and working that way. So, you can either have many queues with some items in each or one queue with a whole bunch of items.