Lets for example say I wanna have my server open only for 10 friends + me. I am not sure if the plugin supports whitelist by STEAMID or only works with permission? I am not gonna add this permission to the default group for sure. I have to add it to special group ex. whitelisted and then add my friends to this group or give each of them the whitelist permission but still the same... -> Here's the main problem... Since you need to log in to the server at least 1 time or even be online in order to give permission to the selected player how I can do this and at the same time have a whitelist? Or I need to stop the plugin to allow all of the friends to join and wait for them then load the plugin, give the appropriate permissions/group, or whatever... Then again when someone from these 10 people without a whitelist wanna join I need to do the same to allow him into the server? Cuz not everyone is available to hop in at the same time, but he need's to join 1st time in order to get permission... If you stop the plugin others can also join until it's stopped... ehh?

You can grant permissions in Oxide by steam ID. Granting the permission whitelists them.

But I cant add this plugin and whitelist people since with this plugin they can't join... If they do not join won't be able to give permission or if use oxide.grant user STEAMID can be added to players when they never had joined into the server?

Yes, you can grant permissions without them joining by using their Steam ID.

Thanks.