This should be a core element of WP multi-site functionality, but I haven't found a plugin that simplifies the task.
There are plugins for creating granular control of user roles, membership plugins and spam management plugins, but seemingly no plugins which are up-to-date for the simple task of placing all new sign-ups in a moderation queue enabling super-admins to approve them and/or just throttle the rate of new sign-ups to manage the dilution effect from new blogs or conversations.
I currently have 'Register Plus Redux' installed but there are various bugs - such as assigning a temp email address to new sign-ups which creates an error on approval, or not being able to disapprove a new user from the site admin pages, instead needing to go to the Network user pages in order to delete a user.
Am I missing something? How are other multi-site networks managing user sign-ups? Is there a secret function or plugin that those 'in the know' are using?
Answers on a postcard, please..