Clicking on it will let you invite others to collaborate with you on your map.
I'm not sure how long that feature has been there. (It was probably already there when I wrote my last post.) For now I'll just fool myself into thinking that Google liked my idea and decided to add this feature because of me. :)
It's worth noting that collaborating on a custom Google Map still requires users to register for a Google Account, something that requires an e-mail. If you'd like your students to be able to work together on a map without having to register for a Google Account, Linda Dierks suggests ScribbleMaps.
Thanks for the idea Linda.
As an alternate way to have students collaborate on maps, I've been using Scribble Maps (http://scribblemaps.com). It gives many of the same features Google Maps can without having to set up forms. You can save the map you create to a unique URL and password (still no ID needed) and share it with others. They can save changes as long as they have the correct URL and password.