Sitefinity license is domain based, which means that with one license you can run website on only one domain. However, subdomains are not limited, so you can achieve this through subdomains.
If your main site has the domain: www.mysite.com
you can have your users opening minisites on john.mysite.com, kelly.mysite.com and so on.
I hope you'll find this information helpful. Let us know if there is anything else we can do for you.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.