Hi Billie
Actually, it is quite simple to setup. You just need to:
1. Config Membership Pro so that when users subscribe for the plan, they will be assigned to the Joomla user group you want (like Subscribers or Paid Members , choose any name you like), see
membershipprodoc.joomservices.com/joomla-groups-integration
2. Create a new access level called Subscribers, choose the Joomla user group above for that access level
3. Set Registration Access setting of event to the access level above
After that, it will all work as expected
Regards,
Tuan