Thanks guys. Very helpful.
@Rui, this is kind of what I was getting at.
UserInfoProvider.GetUserInfo takes a username argument. If I use email address for username I presume this means I can't have two different users on two different sites using the same email address for username?
Or do I take it from Brenden's point, that UserInfoProvider operates in the context of the current site in which can it'll only pull users from the current site?
Thanks as always for your advice.