Login to Accompa using an account that has 'Administrator' privileges.
Click Settings on the top-right, and select Manage Users from the menu.
The resulting page shows how many user licenses you've purchased and how many you're currently using. Make sure you have enough user licenses to add the new user.
Note: During "Free Trial", you can add up to 5 users.
Click Add New User Account link under "Manage User Accounts" section. Then follow on-screen instructions to add the new user.