A user with the CreateTicket right can create a ticket in a queue. RT also checks this right when you try to move a ticket into a queue (see info in ModifyTicket).

This right can be granted to users and groups on a global level or on a queue-by-queue basis.

This right should generally be granted to the Everybody group, so that even an unknown user can create a ticket by sending E-mail to the system.

See also: Rights, GlobalRights