What you guys have to do is a factory reset of your Slingbox. After that go to the watch section of the Web SlingPlayer, then go to the Directory, point to your Slingbox, and then point to the arrow right next to it. Click 'Delete Slingbox' or the X to remove it. You may have to do another factory reset afterwards before you can re-add it back on to your Slingbox account. After your done with the setup process, the invite should be removed. It worked for me.
I had invited a friend to watch my slingbox and now I see his email jumbled on the 'Add guest' list and I cannot remove his email so he can still access it! Here is the screenshot http://prntscr.com/bkrii4
When I click on the cross it does not do anything and it stays on