It all seems to work, there’s a (beta) Render button on the website now.
I’m an early adopter hopeful, yes impressive, at least for me.
For some reason my password from my purchase did not work so had to reset it.
There are different ways to get it working from Dorico, including this one described in another thread.
For me, I exported musicXML from Dorico (edit: current version 5 of Dorico does not export midi with lyrics which Cantai web render needs), imported into Musescore. Tidied up there (I had to separate my divisi as it does not do this), export midi, on the Render page render it. Two tracks appear, one dry, the other based on reverb settings you can set before rendering.
Richard demonstrating the process (Musescore to Web render) and with Sibelius (linking to them here although the links would be elsewhere I would think.)
I imported the .wav files into a DAW and added my own reverb. (No instruments, just singing.)
The Please Read box and Guidelines on the render page gives very useful and important information.
Slowly various more features and voices will be added we are told.
Richard’s (today’s) YouTube update here titled “We’re LIVE and this is the WORST it’s ever going to sound!” specifying its only at about 25% quality.