There are two types of users in Sublime: Users and Administrators. Users have the ability to access the Sublime web interface, create repositories (depending on security settings), and access repositories they have been granted access to. Administrators can do all of the above, but they also have access to the Administration section of the web interface. Thus Administrators have the ability to create users, manage server settings, take the server offline, schedule backups, etc.
All user management is done from the Users tab in the Administration section. To create a new user, or edit an existing user, simply click the Users tab in the Administration section, then select the appropriate option.
Once a user is created, he or she will be able to edit their email address and change their password. They will not be able to change their username however.

Note: If you are using LDAP Authentication, you cannot create new users. You can however click on existing users and make them Administrators.