Licenses in Epicenter Server are based on the number of mailboxes. A mailbox will only be counted once, even if it appears in multiple tasks. If you distribute to an Exchange distribution list, each recipient mailbox will be assigned a separate license.
Licenses will automatically be assigned as long as there are licenses remaining. Therefore there's no need to manually assign user licenses when you configure Epicenter Server.
Licenses will continue to be assigned even if you remove a mailbox from tasks because a license can only be deassigned if the mailbox is deleted. You can manually disable a license for a mailbox under Help | Manage Licensed Users, but you cannot reenable a mailbox once it is disabled. For more information on the Manage Licenses window, see the Help menu page.