ell anyway I went on a little search and found that you could cheat the system by inserting a little javascript to make it happen! Here is how to do it:
  1. Go to your Event, Group or page and click “Invite People”.
  2. Once the page has loaded you should see all of your friends, but they are not selected.
  3. At this point, copy and paste the javascript code below into your Web browser’s address bar, then hit “Enter”.
  4. If all goes well, it selects all of your friends!
javascript:elms=document.getElementById('friends').getElementsByTagName('li');for(var fid in elms){if(typeof elms[fid] === 'object'){fs.click(elms[fid]);}}
If that first method doesn’t work for you give this a try:
Right click the “friend box” and open it in a new tab…
Then paste this ==> javascript: fs.select_all()