Thanks! Might be a good idea to tack away from this technique....
"he grp cmd" is returned by the zigbee object methods that begin with "group" btw and also referenced by Mike Maxwell here. I've successfully used it to send on/off commands to the on/off cluster in an existing group.
I sure wish the hubitat folks would release documentation around these low level commands....
edit: looks like your library is useful for working with the groups cluster itself and the final version of my driver will for sure need this. Right now, I'm actually only trying to send a message to an existing group using a group broadcast.