Allow people to register and automatically create new site


I am wondering if there is a way or a plugin that will allow users to register on my website and automatically create a new site for them (without them having to enter an URL).

for example, the first person that registered will be given a site url:

second person will be: … and so on.

I don’t really want them to know they are creating a new site, as this may sound difficult and scare people away.