If you're an invitee, it means that the group purchase is not happening. Your card will not be charged. If you don't get an email explaining what happened to the group, you should contact the group's organizer for the details.
If you're an organizer, you'll only get this email when you're using Pay By Group through one of our partner businesses, and that business needs to cancel your group for some reason. They should always get in touch with you before this happens -- if you get this email unexpectedly, please contact firstname.lastname@example.org. For more, see this article.