There is no opportunity to add a user for an existing subscription. You should cancel all your subscriptions and make it over again for 2 users at once (in the amount of 10 dollars for two users) . After this you can invite one user to your app because you are the first user of your app.
How do I get training for building my app?
Why don’t I see my changes in the preview app?
Do invited users have to choose between development and production versions?
What is the difference between development and production data?
How does email function work?
How does SMS feature work?