Automatically Select From The Drop Down Menu The Group Associated With That Particular "join This Group" Link

I'm a relative novice at java script. I've been working on a google map that can be found at: [URL](way too much code to post here). Each of the makers on the map has s particular set of information that pops up in a window when it is clicked, including a hyperlink called "join this group." Right now clicking on that link merely takes the user to the form at the bottom of the page.

What I would like the link to do is to both take the user to form at the bottom of the page and automatically select from the drop down menu the group associated with that particular "join this group" link. For example, if a user selected the marker representing the Neighborhood, Beverly group and then hit the join this group link, the drop down menu would automatically select the Neighborhood, Beverly (Pless) for them. I suspect this involves using java script to pass a value. I just have had no luck getting it to work.