Configure user settings at Control Panel → Configuration → Instance Settings. In the Configuration tab, scroll down to the Users section and expand it.
The Users section has three tabs: Fields, Reserved Credentials and Default User Associations.
In the Fields tab, enable/disable these fields on the Add/Edit User Form:
Autogeneration of screen names
In the Reserved Credentials tab, specify the screen names and email addresses Users aren’t allowed to use. This prevents Users from creating IDs that look like administrative IDs or that have reserved words in their names.
Default User Associations has four fields for listing (one per line) the Sites, Organization Sites, Roles, and User Groups you want all new Users assigned to automatically. By default, new Users are assigned to only the Users Role. You can also choose whether to apply these configurations to existing Users.
User groups are handy tools for pre-populating your Users’ private Sites with pages, assigning Roles and permissions, and managing site membership.