After adding the user to the organization, you must assign a role to that user. This can be done by following the steps below.
1. Click on your e-mail ID on top right corner.
2. Select My Organizations.
3. Click on Apps on the right side of the window.
4. Choose the Settings of the app you want to give access to.
5. Click on Roles tab.
6. Select the user and assign a role (Admin, Developer, Customer Support) from the drop down menu.
7. Click on Add to finish assigning a role to the new user.
For more information on app-level roles, read this article.