Nokorthom,
You have 2 divs which are set to hide any overflow from contents they contain. Since the Invite module expands and gets wider than the area you're showing it in, these overflow tags can hide this. There's no way for us to re-center the pop-up, unfortunately, or we would.
If you make the following changes to your CSS files, it should fix your issue:
social/templates/beez_20/css/layout.css, line 450:
* In the .item-page class, remove the overflow:hidden tag, or change it to overflow:visible
social/templates/beez_20/css/position.css, line 56:
* In the #contentarea, #contentarea2 divs, remove the overflow:hidden tag, or change it to overflow:visible
With the 2nd change, you'll see the Invite module overhang the page a little. Again, there's nothing we can do about that.
Hope that helps, and good luck,
Alex