Adding member.entity to Group resource

Resource = Group
We have created a Group resource with a member.entity referencing patient A. At a later time, we need to add a new patient, patient B, to that same Group resource. How is this done without affecting the reference to the original patient, patient A? We want an end result of the Group with 2 member.entities (reference to patient A and reference to patient B).

Perform a PUT against the Group, transmitting an instance containing multiple member repetitions - one for each patient. Alternatively, you could use PATCH.