After adding the user to the organization, you need to 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 the "Apps" button on the right side
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
For more information about app level roles, consult this article.