You cannot directly assign a service to a user or a user to a service.
As an entity manager, if you decide to organize your entity into mailboxes , you can assign services and add users to a specific mailbox. In this case, all users in that mailbox will be able to access and work on the assigned services.
If you do not use mailboxes in your organization , all your users will have access to all available services, without the need to assign them.
Only if you choose to Use mailboxes in your entity , you add users and assign services to specific mailboxes that you have created. If you do not use mailboxes, no assignments are required, as all users will have access to all available services.