How to manage users and roles?
Users are managed through Settings -> Users. On the Users page you can add new, edit existing and delete users from the system (as long as the user you are logged in as has the appropriate permissions)
Roles are managed by going to Settings-> roles. On this page you can add, edit and delete roles from the system.
@yeasuh - Just go to Splunk UI > Settings > Users to manage users and Splunk UI > Settings > Roles to manage roles. (Splunk Navite users and roles)
FYI, you could also have integration with LDAP and SALM for users and roles.