Add REST Resource for Creating Order Group.
It should Create Multiple Orders within a Group in one API Call. Since order is not updatable it can only create an order.
Additional rest resource for "Group Order" might be GET, VOID and DELETE Group.
Here is the link for the Discussion:
The underlying OrderGroup class was introduced in openmrs-core 1.12.0, so this resource should be available when core 1.12+ is running