Subsite User access

I was wondering if there was a way to give editors access to subsites only. For example, can I create http://user.mywordpressblog.com and only give them access to that “wordpress blog?” without ever allowing them to edit http://www.mywordpressblog.com?

Would this mean I would be able to create a different wordpress blog installation under http://user.mywordpressblog.com?

Thanks