Adding admins to your RUST server is quite simple.
Go to http://steamid.co/ and search for your Steam profile name. After that, you should see the following:
Copy your "Steam 64 ID" . In our case, it is 76561198028178387
Click the "Add Admins" button on your game panel and then fill out the fields. After you are done, click "Execute" and restart the server for the changed to take affect.
To remove an admin, go back and select "Configuration Files". From there, edit the server\fragnet\cfg\users.cfg file, and remove the lines of the admins that you want.