APFS volume group creation

create-volume-group.sh

# Lookup your existing disks and volumes
diskutil apfs list

# We will modify diskX
disk=diskX

# Assuming that the volume diskXs1 already exist, we need to mark it as a data volume before adding it to a group.
diskutil apfs chrole ${disk}s1 D

# The volume group cannot be created directly. It will be created during the creation of the 2nd volume.
diskutil apfs addvolume ${disk} "Case-sensitive APFS" MyVolumeName -groupWith ${disk}s1 -role D

# (to create a system disk instead, set `-role S` in the above command)

# Now the volume group should show up
diskutil apfs listGroups

# Note: the Disk Utility desktop app does not always refresh correctly. Restarting the app or unmounting/reconnecting should help.