Final answer:
The leader introducing a nurse to the support group members and advocating for their professional relationship is performing the function of an advocate, reflective of expressive leadership.
Step-by-step explanation:
When the leader of a support group introduces a nurse to its members and discusses the professional relationship with the nurse, this leader is performing the function of an advocate. Being an advocate in this context means the leader is supporting and promoting the interests of the group's members by bringing professional resources like a nurse into the group. This function aligns with the broader goal of providing emotional support and health benefits to the group, which is characteristic of an expressive leader, one who focuses on promoting emotional strength and health among members.