Embedding dropdown menu with custom datagridview

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C#

DataGridView

drop-down

I have created a windows application which retrieve data from .dat file and convert it into datagridview(as per my requirement, it splits data into columns).Now column named "Grouptype" is there. I want to create a dropdownbox in that particular column. and it should be filled with the existing data of that column.
E.g. if there are 5 different values for Grouptype column then the dropdownbox should be filled by those values. and if i add any new row and grouptype column has any new value then it should be allowed to enter.

For binding purpose i have used following code.

public static SortableBindingList<User> LoadUserListFromBkpFile(string path)
        {
            var users = new SortableBindingList<User>();
            char[] delimiterChars = { '|' };
            foreach (var linedata in File.ReadAllLines(path))
            {
                string[] words = linedata.Split(delimiterChars);
                if (words.Count() < 10)
                {
                    return users;
                }
                if (words[0].Trim() != "")
                {
                    users.Add(new User
                    {
                        LineNumber = int.Parse(words[0]),
                        GroupType = words[1],
                        CountyCode = words[2],
                        County = words[3],
                        Payee = words[4],
                        Address1 = words[5],
                        Address2 = words[6],
                        City = words[7],
                        State = words[8],
                        ZipCode = words[9]
                    });
                }
            }
            return users;      
        
        }

I don't know how to add dropdownbox with this. and fill it with existing data at run time.