sooo Im using this in conjunction with Tebex but at the same time I have tested this in just RCON

In Tebex Im using
addgroup {id} donator 30d
which dosent work at all, at least RCON dosent respond

Now I also tested with just using RCON. I used:
addgroup fullSteamID donator 30d
and RCON didnt reply and the user was not added to donator group

Make sure the plugin is loaded or check using "plugins" command if there are any errors.

Try direct input to the server if possible.