Focusing at night can be a difficult job. Of course it you have a great bright moon it is simpler. In general I suggest that you turn off the auto focus (either on the camera or lens- I prefer to do it on the camera). Now look for the brightest thing a long way away. It can be a light on the ground at least a few hundred feet away, the moon, or bright starts. I normally do not have any assistants but I have heard of photographers sending one out across the fields and then shine a small LED flashlight back at the camera. It works, but I am not sure I like it as you might loose your assistant in a large dark hole or anger some other photographer by flashing his shot.
Here is how I do the job:
With camera set to manual focus, switch to “live view” and pre focus the lens at infinity (use the center of the infinity symbol on lens, not the end of travel of the focus ring). You need to have the camera on a tripod. Then put the brightest thing you can see in the sky (or a light on the ground several hundred feet away) and center it on the live view screen. Now click the magnify button a few times to make it bigger and then adjust the focus for a sharp dot of light. This works on my Nikon and I think works on Cannon, Sony, and others, but do not know.
Check the lens and make mental note as to where infinity is for future reference. Then it is a good idea to put a piece of tape on the lens to keep you from twisting the focus ring in the dark by mistake.