We do not provide member registration and login features yet. A membership service has been a long requested feature and we are excited to finally open up the discussion to better understand your requirements. We encourage you to complete our survey as your valuable input will enable us to ultimately deliver the best solution for your business / website.
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.