[otrs] Sub Queue Related Query

Hi, I have created a queue by name Helpdesk and queue belongs to Helpdesk Administrator group. I have created several queues such as Servers, Lab, Telephone, etc. under Helpdesk queue. All these sub queues are put under Service Department group. Now, I have noticed that, customers are able to see and submit their tickets to sub-queues also. How to hide these sub queues from them. How to configure OTRS for assigning a ticket to 3-4 members at a time. I have tried by putting these members in a group, but not able to assign the calls. I don't where I am missing some config settings. Thanks and Regards, Prafulla Kumar