To purchase an Engineer, place your cursor over the numbers above the "Construction" button, click "Purchase Engineer", view the information about the price in the pop-up window, and then click "Purchase". NOTE: The number of Engineers you may purchase is limited. Once purchased, they cannot be removed from your Sector.
Engineers are special workers that assist you in constructing or upgrading your Sector's buildings. Each additional Engineer adds an extra construction or upgrade queue to your Sector, allowing you to upgrade or construct multiple buildings simultaneously.