Add an unlimited number of collaborators to your site with just their email.
How to Add Users
From your The.com Profile:
Click the arrow icon next to the "Edit Site" button of the site you want to share.
Click the "Invite" option.
Type in an email address. If there is no account with a matching email, it will send them an invitation to create an account.
Click the "Invite" button.
An email will be sent letting them know that the site was shared with them along with a link to edit that site.