select-many-with-parent.cs

var results = users.SelectMany(p => p.Departments, (user, departement) => new Tuple<string, string>(user.Name, departement.Name));

foreach(var userAndDepartment in results)
{
    Console.WriteLine(string.Format("User: {0} - Department: {1}", userAndDepartment.Item1, userAndDepartment.Item2));
}

Console.ReadKey();