On the subscriptions page, you would use the + at the side of any of the subscriptions to add a member.
To be honest, the issue here is actually the member who can access, as they shouldnt be able to. Your system is actually not allowing me to log in as members, so if you would like me to check that I need to disable all 3rd party items, including the application you have set as default. If you would like me to go ahead with that, please let me know. The correct way in which to do this however is to add a subscription