We've set this up for many of our "do it for me" (http://www.paidmembershipspro.com/pricing) customers as a custom feature. Some version of this may be released in some form in the future. It is a common request (but often requires specific features for each site).
One manually work around is to create to levels. One "pending" level and another "members" level. Have people sign up for the "pending" level and then switch them over to the full "members" level manually after you review. (This can be complicated if you want to charge people a recurring fee at the time the checkout for the pending level.)
Hope this helps a bit.