logically what you want to do is not possible unless records inside the group use the same email, otherwise when you sort by email it splits up the group.
if that is the case make a formula field to concatenate the email and other field together and gropup on that.
Otherwise you have to make a determination of which email in the group becomes the only email for the group that is relevant and then you can likely sort the groups using a group sort, not a field sort.