Ed,
There's a few things that could be causing that. First, the Invite module uses some Facebook code, which frankly, isn't very good and can get chopped off. In the module parameters, there's a setting for Enable Autoresizing, which we'd recommend you try. It's our way of fixing some of Facebook's issues.
If that doesn't work, then there may simply be a conflict between using the RokBox and JFBCInvite. If RokBox doesn't automatically resize itself when the JFBCInvite module does, then even if 'we' try to resize our module to make sure the Send / Cancel button is visible the RokBox may still clip the window.
Hopefully, the automatic resizing setting will fix it, but if not, let us know, and we'll see if there's anything else you can try. The next suggestion would be to try to modify RokBox to hard-code it's width to the widest setting that JFBCInvite could need.
Good luck,
Alex
Facebook Invite
