A FusionAuth Group has a few use cases.
The first use may be to simply logically group one or more users within a Tenant. Once a User is a member of a Group they may be identified as a member of the Group and retrieved using the Group APIs.
The second reason you may wish to use a FusionAuth group is to manage Application Role assignment. A Group may be assigned roles from one or more Applications, a member of this Group will be dynamically assigned these roles if they have a registration for the Application.
For example you could create a Group called
Admin, and assign this group the admin role from each of your applications.