How to Force Admin Approval of New Users?
This Article used to recommend the New User Approve plugin. However, that plugin has not been updated in quite some time, and although it does still work with the latest WP Versions we can't recommend it any longer as it could pose future compatibility issues with WordPress. Instead we now recommend a new 3rd party add-on found here.
Though MemberPress itself does not currently have a Member approval system in place, there is a new 3rd party add-on which handles it quite nicely.