How to authenticate user in Spark
Here is how to add new user accounts* in the Spark interface (if you don't want to use the URL that Noah provides above).

Step 1: Click the down arrow icon at the upper right of your screen.



This will expose another menu.

Step 2: Click "Manage Users" on that menu. 



This will take you to the the screen where you can add new users.

*It might be helpful to review the difference between CiviCRM contacts and user accounts.