I would suggest sending an email to them from your account letting them know that they've been un-enrolled from your group as either their email application or ISP thought it was spam.? Sending it from you account and not via Groups.io is an attempt to get the email delivered, if their provider considers Groups.io email as spam.
Generally, the message sent from Groups.io to notify them that they have been removed is in their spam folder where they may never see it.
Regards,
Mike
