Ok, I've written an example which should sort out out. Just a note that this is not really a Sharepoint related question but more a Linq/Data Question. Anyways, this should do it :
internal class Program
{
private static void Main(string[] args)
{
DataTable demodatatTable = CreateDemoDataTable();
var JohnTasks = demodatatTable.AsEnumerable().Where(z=>z.Field<List<string>>("Users").Contains("John"));
Console.WriteLine("John Tasks : " + JohnTasks.Count());
Console.ReadLine();
}
private static DataTable CreateDemoDataTable()
{
System.Data.DataTable d = new System.Data.DataTable("MyDataTable");
d.Columns.Add("Task", typeof(string));
d.Columns.Add("Users", typeof(List<string>));
List<string> users1 = new List<string>();
users1.Add("John");
users1.Add("Peter");
d.Rows.Add("Task1", users1);
List<string> users2 = new List<string>();
users2.Add("John");
users2.Add("Peter");
users2.Add("Sam");
d.Rows.Add("Task2", users2);
return d;
}
}