iphoto deletes email addresses added to shared photostream

My up-to-date iPhoto will not "take" email addresses (Gmail addys, btw) I add to the invited/subscriber list of a shared photostream. I put them in by typing part of a name which properly autocompletes, but when I click away to a different photostream or another folder in the Source List, and then click back again, they are gone. Anyone having a similar problem, or a solution?