How to register sites for user in multisite

Am I missing something – as admin of my network I’d like to be able to register a site FOR an existing user. I don’t see a way to do this – I can of course create a new site but then it’s attached to me and not to a different user.

Also, how do I allow existing users to create a new site. I know how to allow them to create one at the time that they’ve signed up. But after they’ve signed up, the signup.php url no longer is available, nor is the url. So how can a)create a site for them from the admin and alternatively have them register for a site after they’ve already signed up as a user. I have the network settings to allow both signups and allow users to create a site. And I should add, I need to be able to user a blog template in both scenarios.