Each person should buy their own policy rather than relying on a single group one, and buy it when the first non-refundable deposit is paid — cancellation cover generally only applies to what you've already committed. This is guidance, not advice: read the policy for your own situation.
Group trips have more moving parts, and the risks aren't shared: one person's medical claim, another's cancelled connection, a third's stolen phone. Individual policies mean each person's cover matches their own circumstances, existing conditions and credit-card benefits.
The scenario most people don't insure for is one person cancelling and the rest still going — which can leave the group liable for an unfilled accommodation share. Read how the policy treats that, and pair it with a written cancellation rule in the group.