Same. I went to the link from the Tweet and it worked fine. I created a new group of 4 people and it worked. However, just visiting duo.google.com as you stated does not have the group feature listed, at this time.
Doesn't seem to be working for me yet. I did update to a new V83 and still no dice.
Edit: it allowed me to make a group with 12 total people if I followed the link in the tweet.
https://duo.google.com/about/?noredirect&gc=1
Click try duo and the button should appear.
Well I just tested it with 3 others and it didn't work, maybe it's still rolling out.
Side note: the Group Call option is there (on the left hand side) if you follow the link in the tweet, if you browse to duo.google.com the group call option is not there.
Not sure what your situation is but on Windows there is no Duo app. It's just something that runs in a browser (Chrome or Firefox). So if you don't have duo.google.com and logged into your Duo account on in the browser, the call cannot be answered there. I have the same Duo account on a number of devices (phone, Lenovo Display etc.) and they are pop up the call even if the app isn't running. But the PC requires is to be running.
I did have the gmail account setup on a Windows computer at duo.google.com first.
The overall experience is jank. People can not find him as it only looks up by phone number, but he can call others if their numbers are used. Also I can make group calls with him in it.
You need an app that disables the touch screen (but will recognise specific swipes).
I used Touch Screen (https://play.google.com/store/apps/details?id=com.kidscrape.touchlock.lite) in the past. It's now a version that may need a monthly payment, according to the reviews though.
This number likely came from the playstore count:
https://play.google.com/store/apps/details?id=com.google.android.apps.tachyon&hl=en_US
​
That said, just because someone downloaded Duo, doesn't mean they are a "user" not by a longshot.