public class Order
{
     [Key]
   public Guid Id { get; set; }
    [Required]
    public int Type { get; set; }
    [Required]
    public int Day { get; set; }
    public Guid? GroupId { get; set; }
    public Group Group { get; set; }
    public Guid? UserId{ get; set; }
    public User User { get; set; }
}
public class Author
{
    [Key]
    [ForeignKey("User")]
    public Guid Id { get; set; }
    public virtual User User { get; set; }
    public virtual ICollection<Group> Groups { get; set; }

    public Author()
    {
        Groups = new List<Group>();
    }
}
public class User
{
    public User()
    {
        Groups = new List<Group>();
    }
    [Key]
    [ForeignKey("Author")]
    public Guid Id { get; set; }
    [Required]
    public string Login { get; set; }
    [Required]
    public DateTime BirthDay { get; set; }
    [Required]
    public string FirsName { get; set; }
    [Required]
    public string SecondName { get; set; }
    [Required]
    public string Password { get; set; }
    [Required]
    public string AvatarPath { get; set; }
    [Required]
    public string ResponseQuestion { get; set; }
    [Required]
    public string SecretQuestion { get; set; }
    public virtual ICollection<Group> Groups { get; set; }
    public virtual Author Author { get; set; }

    public static implicit operator Author(User usr)
    {
        return new Author()
        {
            User = usr
        };
    }
}
public class Group
{
   public Group()
    {
        this.Users = new List<User>();
    }
    [Key]
    public Guid Id { get; set; }
    public string Name { get; set; }
    public Guid? AuthorId{ get; set; }
    public virtual Author Author { get; set; }//Автор группы
    public virtual ICollection<User> Users { get; set; }

    public Group(string name, Author author)
    {
        this.Name = name;
        this.Author = author;
    }
}
public class StoreDB : DbContext
{
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
            .HasOptional<Author>(a => a.Author)
            .WithRequired(a => a.User);
    }

    public DbSet<User> Users { get; set; }
    public DbSet<Group> Groups { get; set; }
    public DbSet<Order> Orders { get; set; }
    public DbSet<Author> Authors { get; set; }

    public User GetUser(string login)
    {
        return this.Users.FirstOrDefault((usr) => usr.Login == login);
    }
    public void AddUser(User usr)
    {
        this.Users.Add(usr);
        this.SaveChanges();
    }
    public void AddGroup(Group grp)
    {
        this.Groups.Add(grp);
        this.SaveChanges();
    }
    public bool IsContainUser(string login, string pass)
    {
        try
        {
            this.Users.First((u) => u.Login == login && u.Password == pass);
            return true;
        }
        catch
        {
            return false;
        }
    }
    public bool IsContainUser(string login)
    {
        try
        {
            this.Users.First((u) => u.Login == login);
            return true;
        }
        catch
        {
            return false;
        }
    }
    public bool ResponceOnQuestion(string login, string response)
    {
        try
        {
            this.Users.First(u => u.Login == login && u.ResponseQuestion == response);
            return true;
        }
        catch
        {
            return false;
        }
    }
    public void UpdatePassword(string login, string newPassword)
    {
        User updateUsr = this.Users.First(u => u.Login == login);
        updateUsr.Password = newPassword;
        this.Entry<User>(updateUsr).State = EntityState.Modified;
        this.SaveChanges();
    }
}

<add name="StoreDB" connectionString="data source=(localdb)v11.0;Initial Catalog=ChudoPechkaStore;Integrated Security=True;"
providerName="System.Data.SqlClient"/>