All you need to know in order to add a user to a site is the email associated with their The.com account.
From your The.com Profile:
Click the gear icon at the top right of the site you want to share.
Select the People tab.
Type in the email address of another The.com user.
Click the Invite to Site button.
An email will be sent letting them know that the site was shared with them along with a link to edit that site.
Each site allows up to five members without a fee. After all five seats are filled, each additional seat costs $5.