Got it!
For anyone facing the same problem:
Download a telnet tool which supports RAW connections (cmd telnet sometimes won't open RAW connection) - use PuTTY for example,
Login via telnet, your server IP:Port and server admin password
example:
Make sure the player you want to add to the admin group is logged into your server (AFTER YOU connected via telnet)
You should see a message for the successful login and can use this information to get the exact player name (and platform IDs, but these didn't work for me)
Type the following command:
admin add "playername" 0
(you've to type the name with the "" )
Done