Unit Testing a Domain Model Containing Lists Populated From The Database

I am currently in the middle of writing unit tests for my domain models.

To lay a bit of context I have a Role Group class which has a list of Roles, it also has a list of Users that currently have this Role Group assigned to them.

A Role Group is assigned to a User so all the methods to do this are in the User domain model. This means the Users property on the Role Group side is basically pulled in from the database.

Both Role Group and User are Aggregate Roots and they can both exist on their own.

Unit Testing a Domain Model Containing Lists Populated From The Database

What I am struggling with is I can not test the CanArchive method below because I have no way off adding in a User to the property. Apart from the bad was of using the Add method which I don't want to use as it break the whole idea of Domain Models controlling their own Data.

So I am not sure if my Domain Models are wrong or if this logic should be placed in a Service as it is an interaction between two Aggregate Roots.

The Role Group Class:

    public bool Archived { get; private set; }

    public int Id { get; private set; }

    public string Name { get; private set; }

    public virtual IList<Role> Roles { get; private set; }

    public virtual IList<User> Users { get; private set; }

Updating Archived Method:

    private void UpdateArchived(bool archived)
    {
        if (archived && !CanArchive())
        {
            throw new InvalidOperationException("Role Group can not be archvied.");
        }

        Archived = archived;
    }

Method to check if Role Group can be Archived

    private bool CanArchive()
    {
        if (Users.Count > 0)
        {
            return false;
        }

        return true;
    }