Knockback is proportional to damage dealt (more damage = more knockback). This also means that knockback is affected by location-based damage, too. So if you want to boost your teammates most efficiently, try to shoot them in the head (or at least not the legs), and make sure you fully charge your pistol.
Sometimes when I'm playing cargo O HW my teammate will shoot a half-charged pistol shot at my legs, which doesn't give me much speed. It would be better to shoot a full-charged shot at my head for maximum speed. Just charge your pistol all the way and aim a little bit up.