Does it impact a site's Google ranking if I link to the site with a "referrer" query-string? e.g:
example.com/?ref=mysite instead of
As long as that url redirects to the main url (and wipes the referrer parameters itself, which you can check by just following the link) or that parameter is set in google webmaster tools to be ignored by googlebot (which is only easy to find out if you're the owner of the site, really), it should make essentially no difference.
Otherwise you could resort to using the
<link rel="canonical" href="http://www.example.com/"> (again if you are the owner of the site being linked to).
The question mark is normally a signal for dynamic content, which may send signals to the robot ... some dynamic sites loop into infinity with different urls all with the same content.
Inherently example.com/?ref=mysite example.com/?ref=mysite&anothertag=something example.com/?ref=mysite&anothertag=Duplicatecontent More than likely will all pull up the same content while example.com/mysite would signal static content and example.com/mysite/anothertag would be different content.
Currently many CMS systems use ? marks and they are indexed by google. None the less, they are by nature not search engine friendly. Duplicate content is a problem for robots as well as url loops that trap search engine spiders.
I would strongly discourage using ? but I can not they will specifically harm a sites ranking. Ultimately the URL with the ? will be given importance by links going to it from the site and other sites in the same way that none ? URL are given importance. The ? mark itself does not create a penalty, rather opens a can of worms regarding duplicate content and duplicate content is a problem.