How to share my app with App Manager?
iTunes Connect is a separate service from the Apple Developer portal. Development privileges are not assigned via iTunes Connect.

Additionally, since you are using an individual developer account, there is no way to grant another registered Apple Developer access to your team. That functionality is only available to Organisation account.

The problem can be solved in part by granting your colleague access to your code repository. That way you both can collaborate over code. If you want him to test the app on device, you can choose to export your identity on to his machine (Recommended only if you trust him with this thing).