We do not provide member registration and login features yet.
In the meantime, you can password protect pages using our Password Protection feature, which allows you to create a private website, or make part of your site private, by password protecting certain pages on your site. Take a look at the Password Protection tutorial to get started.
Alternatively, you can create a separate account for the site you would like to collaborate. This way you can keep the shared site separate from any other sites in your account.