After some experimentation and research for http://forum.goatech.org/t/grovepi-basic-setup-reading-sensor-values/96/3 it seems there is no simple way to send data from Scratch to the web.
I posted in the Scratch forums to see if anyone else knew of a way https://scratch.mit.edu/discuss/topic/292874/ and got a couple responses.
The first recommended http://scratchx.org/ which has lots of interesting projects but nothing that seems to do what we want out of the box.
The second is someone who is working on “a GrovePi Scratch 2 extension”. They have hacked together a solution using Node-RED that I am waiting to try out.