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.
In game, press F1 to open the console.
Type admin add [Username] 0 replacing [username] with their username.

The targeted player will now have admin permissions.
This will give you a list of all online players, and their IDs

Type admin add [ID] 0 replacing [ID] with their ID.

The targeted player will now have admin permissions.