Updated Thu, Nov 10, 2011 by Martuk
We've investigated this issue further overnight, and I've got an update for you. It looks like a small number of people got stuck in a unhelpful state within our database. That means the problem's on our side, not yours, so don't worry about trying different browsers or similar to fix it yourself.
Our DB team is tracking down the affected accounts and should roll out a fix later today. When that happens, you'll be able to accept the invite. We'll update this thread when the fix is applied. Thanks for your patience while we resolve this!