I set up a google sites site.
I added some people to administrate it.
I notice that one guy whose email address is his own domain like email@example.com it lets me add his email address.
Another person that's firstname.lastname@example.org it lets me add him
But then when I try to add email@example.com it says "Sorry, you cannot share with firstname.lastname@example.org because they do not have a Google Account."
Well, how does it know whether email@example.com has a google account. And why does it have no problem with firstname.lastname@example.org?!