Is it possible for sites to show up on the IE history list if they
have never been visited?  When pop-ups are x-ed out of, do they show
up as a visited site?  Is there any way for a google.groups site to be
on the history list without having directed the browser there?

Yes it is possible that a site gets entered in to the url history if
it was an advertisment. As far as a yahoo group, that is not likly
unless it was visited. The reason for this is because you generally
have to login to yahoo groups to visit them and they in most cases are
not advertised in anyway exspecially through popup.