o.group add 76561197979861396 blablabla admin​

20:54 [Error] Failed to call hook 'GroupCommand' on plugin 'RustCore v2.0.4139' (FormatException: Input string was not in a correct format.)
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000fb] in <4452f821def6406d834e4149849fe7ea>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <4452f821def6406d834e4149849fe7ea>:0 ​

Protocol: 2151.175.1
Build Date: 02/12/2019 14:53:55
Unity Version: 2018.3.3f1
Changeset: 32026
Branch: release
Oxide.Rust Version: 2.0.4139​

You should only need steam id or player name.

o.group add 76561197979861396 admin​

o.group add blablabla admin​

Thanks

My bad, i go to sleep :D i needed oxide.usergroup add not o.group xD