I don't profess to know if Groupwise will work, but to set the default email client, go to the Mail.app preferences pane General. Go to the drop down, Select Default Email Reader, select Select... and then click on Groupwise if it appears in Applications pane. Again this doesn't mean it will or will not work, just how to change the default reader.
I did try that but was getting a FM error "Some of the files that are needed are either damaged or have not been installed. Please run installer to correct this problem".
The error goes away when I reset the default email back to Mail so I don't think the problem has anything to do with damaged files.
Filemaker Help will list the email clients that are compatible to it if you look up Send Mail. SInce you are a mac user, you can add Microsoft Outlook to the list there if you have the latest release of Filemaker with the latest patches installed.