We delegate group ownership, and it still is puzzling how many times a group admin may enter a person's Internet Address vs. picking the user's canonical name from the directory. It would be good to have some logic to prompt the user with an 'Are you sure?" type question when the are adding a member containing an "@" in the entry. This certainly would make the most sense on General Purpose, Security, and Deny Groups. I'd say just place it for all with a one time checkbox "Do Not Show This Again" on the prompt.