try another way to implement the data stucture

UserService.Infrastructure/DataModels/SecurityGroup.cs

@@ -1,6 +1,9 @@
-namespace UserService.Infrastructure.DataModels
+using System.Collections.Generic;
+
+namespace UserService.Infrastructure.DataModels
 {
     public class SecurityGroup : Member
     {
+        public ISet<MembersMember> Members { get; set; } = new HashSet<MembersMember>();
     }
 }