Great execution, but in terms of the market this strikes me as being 3 degrees separated from profit:

Tweetvite -> Tweetup -> Twitter -> Profit!

There's no way anyone will confuse this with http://twtvite.com/

That's a fair comment. The domain name (as much as some of you are going to hate it since it follows the whole twit, tweet, etc. nomenclature) is a pretty good one. The interface is entirely different if you compare the two side by side, so yeah, it's possible that there might be confusion, but we've done everything that we could to differentiate there.

Nice work Jeff. The UI is really clean and the option to not register is useful.

The post says spin-off. Is it a new startup or a new app?

It's a new app. Our startup is called Simpletastic Inc. and created both Anyvite and now Tweetvite.