public class Member
{
    [Key]
    public string MemberId { get; set; }

    public string MemberName { get; set; }

    public string GroupId { get; set; }

    [ForeignKey("GroupId")]
    public virtual Group Group { get; set; }
}

public class Member
{
    [Key]
    [Column(Order = 0)]
    public string MemberId { get; set; }

    [Key]
    [Column(Order = 1)]
    public string GroupId { get; set; }

    public string MemberName { get; set; }

    [ForeignKey("GroupId")]
    public virtual Group Group { get; set; }
}