gistfile1.cs

public class User
{
  public string Username { get; set; }
  public bool Active { get; set; }
}

public class Group
{
  public string Name { get; set; }
  public List<User> Users { get; set; }
}

If I set Active = false for one user I don't loop all Groups search that specific user to set that property.
BTW I don't what want save the complete User for each Group that it belongs = space waste.