All users get to choose whether or not to allow other users on a shared account to use their sending address or not.
If you want them to have access to your sending address, just click on « sharing » in your sending address settings.
1. Click on your name at the top right of the screen and then select Settings.
2. On the left menu, click on sending address
3. Click on Actions and then Share
If you don’t want to share it anymore, just click on « stop sharing ».