You can add a user to your account through Account Settings. The link pops up when you click your profile in the lower left corner.

Go to the Team tab and type in their email. They'll be able to set their own username and password and have full access to every website in your account.

Team members will have full access to your Action Streams. They can move between website and generate Projects. You can also leave notes on a Project for other team members to view.

Team members do not have access to any billing details.