Fatabjobe,
There's 2 more styles you can try to add, but I'm not sure if they'll fix your problem. I also can't see the module on your page anymore to test these changes, but I can understand if you disabled it temporarily.
In the "s5_modulemm-whiteblue" declaration (~line 1852), add:
z-index: 100;
background: #FFFFFF;
Hopefully, this will give the invite module a background color of white, and when it's widened, make it on top of anything else.
Unfortunately, if this doesn't work, the next best suggestion I'd have is to make it so that Invite module can't overlap into another Facebook widget. They both use iFrames to display their content, and iFrames overlapping can cause some of the issues in your picture above.
Hopefully one of these solutions will help, and not be too inconvenient. Good luck!