[otrs] What customer permissions needed to see own tickets

I am somewhat confused about the permissions model used in OTRS. I have a queue named "Main". One of the settings for this queue is Group which I have set to the group "helpdesk". The "helpdesk" group is a small group of staff who are supposed to answer the tickets. I am not sure what setting the queue Main to this group means (as far as I can tell, this is not explained anywhere in the documentation). But now I have "customers" who submit problems to the "helpdesk" queue but cannot see their tickets when they log in to the customer screen. Why not? Do they have to be added to the "helpdesk" group? If so, doesn't that mean that they could then act as agents and see other customers tickets? Thanks.