With the user manager you can watch what users are registered on your server. The screen looks like this:
If you click on the blue settings button next to a name you can change the password of that user and optionally make him/her a serveradmin by checking the respective radio button. Click on save to make the alterations permanent.
To delete a user on your server, click on the redcross next to the name. Be careful with this function!
With add client you can add a user to your server. Fill in the login name, password and decide if this user needs to be a serveradmin and click on add.