The one I found worked well but it selected everyone, even those who were already members, so I altered it so it would only select friends who needed the invite. Follow the steps below to create the bookmarklet that makes this work. The instructions are for Firefox, but should be similar for other browsers.

How to create a Firefox Bookmarklet The best way to do it would be to add this as a browser bookmarklet. So in FireFox:

1. Right click next to your other bookmarklets and select New Bookmark.
2. In the Name field type: FaceBook Select All (or whatever you want)
3. In the Location field copy and paste the code you see below in the code box exactly as it is. You can leave the KEYWORDS and DESCRIPTION field blank. They aren’t necessary.

   javascript:elms=document.getElementById('friends').getElementsByTagName('li');for(var fid in elms){if(typeof elms[fid] === 'object' && elms[fid].className != 'disabled'){fs.click(elms[fid]);}}

4. Hit Add.

You can also just drag this link to your bookmark toolbar: FB Select All

Now wherever you are adding friends to an event or group using the Facebook friend selector you can click the bookmarklet and it will select everyone who is not already a member.

Enjoy.