Как-то так...
string[] employees = textBox1.Text.Split('\n');
string[] actions = textBox2.Text.Split('\n');
for (int i = 0; i < employees.Length; i++)
{
for (int j = 0; j < actions.Length; j++)
{
if (actions[j].Contains(employees[i]))
{
textBox3.Text += $"{actions[j]}\n";
}
}
}