Create map of US or other area with multiple user locations shown

I would like to create a user-editable map of the US or other large region, where people may be able to click on a location (or enter their GPS coordinates) and display them with markers. Similar to the old thumbtack markers. I am trying to coordinate a meeting of forum users (and their dogs) to arrange for carpooling and ideal locations for such meetings. One idea may be to create a map image with an overlay for the markers. But maybe there is already an application for this purpose.

I’m just starting to search for something and this project sounded good. I may be able to contribute some local data such as trails and small streams and property lines and historical entities near me. I saw that another user added some trails in the Loch Raven Watershed which abuts my property.

UPDATE: I found a cool website that does this quite nicely. Check out:

If you have other ideas I’d appreciate hearing from you. I may add some local data to your project when I get a chance. Thanks!