Inventors on a patent should only be ones who provided inventive contribution that will be part of the material later claimed. It can be very problematic for a company embroiled in a patent contest in court, if an inventor is listed who did not provide inventive input or an inventor was left out who did provide inventive input.
Someone who helps build a portion of the system, but did not provide inventive input to what is described as inventive material in the patent application, is not an inventor.
Normally, the inventors will assign the patent rights to an invention to the company for which they are working for. If someone is an inventor that is not an employee of the company then that needs to be handled properly and their assignment should be obtained. A company should decide whether some incentive should be given to inventors, possibly stock options based on the price of the shares at the time of the invention.