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 bottom left of the screen and then select sending address.
2. Click on "..."
3. Click on Share
If you don’t want to share it anymore, just click on « stop sharing ».