Nothing in HP's documentation talks about how time sync is handled in a stack, so I did a little experiment:
I removed one of my switches from a stack.
I removed the SNTP configuration from the switch.
I changed the time on the switch.
I didn't remove the switch's IP address (frankly, I don't want to have to reprogram via serial.)
I rejoined the switch to the stack.
At this point, and after half an hour of observation, the time on the switch remained incorrect. I therefore conclude that you DO have to assign an IP address and setup the SNTP/TIMEP configuration for every switch you want to sync to your time server.
If this presents a problem for one reason or another (lack of address space in subnet, security issues, whatever) you could set up an out of band IP on the stack members, and bind an interface on your time server to an address on the same subnet.