Additional engineers can be obtained as rewards for completing certain missions, (mainly abduction missions,) monthly rewards for having satellites in Asia, North America, South America and Africa (the more satellites, the more scientists you will receive each month,) and from the occasional council request. You can also add additional engineers by building additional workshops. Once you have a total of 80 engineers, the achievement will unlock. This will most likely come naturally towards the later stages of the game.
TIP: If the achievement does not unlock naturally over the course of the game, start a new game on easy and focus on completing tasks/launching satellites that will net you the most engineers. If given the option between adding scientists or engineers, always choose engineers.