You add users in the Admin Panel, which can be accessed through "Admin" link in the top-right corner of the screen (this link is obviously visible only to users, who are admins for a given Banana Scrum account). Once there just navigate to the "Users" tab and hit the big "Add user" button prominently visible there.

 

A new window will pop-up. There you can provide all the necessary details - login name, full name and e-mail address. Optionally you can assign the user to an existing project or projects. You can also add a personal note to the notification e-mail that will be sent by the system to the newly created user. 

 

Watch this simple tutorial showing how to create users.