Hi,
for sharing personal calendars also working on mobile devices z-push has an additional command parameter/feature named addshared , eg.:
z-push-admin -a addshared -u user_a -n "Kalender\ User B" -o user_b -t calendar -f 751b794e1f2d455586b77096168b56dbc00000000000
It would be very, very nice to have this option also with grommunio-sync . With caldav there are too many deficiency and the z-push enhancement works very flawless. I'm not an php expert but it seems to me that the code modifications are not very complex.
Shared calendar with active sync on mobile devices