Just give them the URLs to the synths you want them to see. That is to say, when you go to a synth's view page, just highlight the address you see in the address bar and paste it into an email, instant message, or other communication to your friend. If you want to link them to a specific photo or part of a photo inside your synth, then you can click the "Share" icon below your synth once you've gotten to the right spot and get a link to that exact view. (Exceptions to this working well are if you are in Overhead view or 2D view.)
Unlisting a synth only hides it from being found in searches on the site, your list of synths page, and Bing Maps.
This means that although no one will be able to find it on their own, anyone who has an unlisted synth's URL can view it without the need for passwords or anything else complicated.