Hi Michael,
The map linked to here:
http://gis.zgroks.com/rw2.html could be used
for the purpose you describe. It allows anyone to add markers anywhere
~ and to delete markers as well. Markers can also be imported from a
pre-existing database (i.e., you don't have to add them manually).
It's based on the Google examples on how to link up to an SQL database
using PHP. Doesn't have the filtering that's including the example you
cite, but that could be added. Feel free to check it out - to add
markers all you need to do is right click on the map. ~ Bob