I've seen other plugins and hacks for this, but for the sake of submitting an alternate method, I've created a post on creating a simple invitation-only hack for signups in MU.
It's features:
- Allows you to set a session phrase in the $_GET that allows users to sign up for blogs, but otherwise allows people to sign up for user accounts without blogs.
- Also allows you as the admin to at least influence what the subdomain/subdirectory will be named by letting you send links to invitees that point to the location you want their blogs in.
Again, this is a very simple hack that just uses the $active_signup variable that's already there in wp-signup.php to affect the needed change. Any thoughts? Does this suck? LOL