First, enable "experimental features" in Site Settings. Then go to the page manager and click on "Create a site with a different address", choose an address and that's it. As the address has the format *.googlepages.com, you can't choose the name of an existing Gmail account.
This is a good way to overcome the limitation of having only 100 MB space and only 100 files, without creating a new account.
The answer to the question: "How do I create a new site?" from help has changed from:
"During this initial testing period, we're only allowing each account to have a single site. However, this site can be comprised of up to 100 pages. We'll soon be offering support for multiple sites, but don't have any specific timeline to share at this point."
"Select the Create a site with a different address link that appears near the top of the Page Manager."