Check the userID exists within the List ?

Im importing CSV file into a list called Students (List<string[]> Students = new List<string[]>();).

ID, First Name, Last Name, E-mail

using (StreamReader reader = new StreamReader(csvPath))
{
    string line;
    while ((line = reader.ReadLine()) != null)
    {
        Students.Add(line.Split(','));
    }
}

//add new item to List
Students.Add(new string[] { textBox2.Text, textBox3.Text, textBox4.Text, textBox5.Text });

above code is successfully save data into Students List. After doing that I wanna add details manually from textboxes and I wanna check the List(Students) weather the List already have a same ID that user enters. if so ask conformation for update List specific record... ?

2nd part

I wanna separate the students for Group wise. Group have to contain minimum - 2 , maximum- 4 students. I don't have a idea for separate them.

Help me.. Thanks