Common ground : Computer A and USB powered Arduino ( MCU ) from Computer B
have 1 computer   


They use a switching supply that is connected to mains ground so the ground of the USB or any of the internal supply connectors is essentially connected to the mains ground


  1 laptop


Also uses a switching supply that may or may have not a connection to the mains ground, in either case the supply is isolated from the mains so a connection of the laptop ground to the mains ground will not cause a problem.


  USB powered arduino ( MCU ).


This is powered from the the laptop so the ground level it uses may be the same as the mains ground or it may not, it depends on the operation mode of the laptop (battery/mains) and the plug that the mains supply of the laptop uses (2-prong/ 3-prong ). In any case connecting the ground of the PC (which is equivalent to the mains ground) to the laptop ground through the arduino will not pose a problem.