Assigns a specific role to group members.
Note: This function is only relevant for site admins, and group members with group admin permissions.
The assignRole() function returns a Promise that resolves to the newly-assigned role after it has successfully been assigned. Assigning a role overrides an existing role. For example, assigning a member role to an admin unassigned their admin role.
Group ID.
Member IDs.
Max: 100 member IDs
Role to assign.