Hello Daniel,
Yes, it's possible to select the group email variable as part of the rejection message of that group. Since the group email is identical to the deleted user's email, you can select the deleted user's primary email variable instead.
The variable availability depends on your workflow setup. In the example below with the User deleted trigger, the trigger outputs the deleted user's email address variable. It's available for the Update group settings action's Default rejected message response field when you click the v button.
I hope this helps.
Thank you.