how do datagridview cell show EF code first forigen key with combobox in winform?

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C#

WinForms

hi I have 2 class in code first

public class Groups
    {
        public Groups() { }
        [Key]
        public int GroupID { get; set; }
        [Required]
        public string  GroupName { get; set; }
        public string GroupDescription { get; set; }

        public virtual ICollection<Users> Users { get; set; }
    }

public class Users
    {
        public Users() { }
        [Key]
        public int UserId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string UserName { get; set; }
        [Required]
        public Int64 DocID { get; set; }
        public string Description { get; set; }
        public int GroupID { get; set; }
        [ForeignKey("GroupID")]
        public virtual Groups Groups { get; set; }
    }

in winform I add datagridview and select datasource to table Users, when I try editing users the GroupID Column not see with ComboBox! how can used this Colmn with combobox to select group??
and so why in datagridview show 2 column GroupID and Groups?