To add a user you'll need to do the following steps:
1. At the top of the page, you'll find Settings > Manage Users.
2. Add an unlimited amount of users and, if you wish, mark them as owners.
Giving someone else admin rights
If you're the admin of an account, a star will appear next to your email, as shown above. To give someone else admin rights as well, simply click 'Mark as owner'.