You cannot assign a service to a user or a user to a service directly.
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 this 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 decide to Use mailboxes in the entity , you add users and assign services to specific mailboxes that you have created. If you do not use mailboxes, no allocation is necessary, as all users will have access to all available services.