Each Nimbus service can be of a different user assignment type that determines how users are associated to this service:
MS Teams-based: Directly tied to your "Teams" the users get automatically added to a Nimbus service
None: For IVR or first-level redirection services
Skill-based: Manual skill-assignment from users you add from within your tenant directory.
Note that the user assignment type is is independent from chosen service type. Depending on which assignment type you choose for your service can access various features.