Group Key with Fileshare encryption will not work reliably with email. You could try and blacklist Outlook.exe and Explorer.exe but there are still ways around it and the file could end up decrypted in transit to email.
My recommendation would be using PGP zip functionality with a passphrase (you can assign a passphrase to a protected zip). Or you can create a single shared key with multiple subkeys underneath of it for each of the users' email addresses and encrypt to that as well.
Does that help?