Normally to add admins you would add players as admins via Steam ID. However with Gamepass you'll need to use one of the following admin commands to give a player admin.
1. Press F1 to open the console
2. Type admin add [Username] 0 replacing [username] with their username.
3. Submit the command and they will become an admin.
1. Press F1 to open the console
2. Type lpi to list player entity IDs and find the player's ID of who you want to add.
3. Type admin add [ID] 0 replacing [ID] with their ID.
4. Submit the command and they will become an admin.