While the above two post are correct, if you still want a owner group, use your rcon password to assign yourself to superadmin first. (Do it to become owner)
rcon_password yourpasswordhere
You can find your pass in the server.cfg.
Once you entered the pass, ulx adduser "ExactUsernameWithCaps" superadmin
Then type !menu, click groups, make owner, make owner inherit from superadmin, aaaand done. Put yourself in owner, Viola.