If you wanted to attempt to use real world physics in this discussion then you would have to take several things into account. First, the lightning would hit the first person, and as kreynolds stated, head straight for ground. Second, we would also have to take into account what kind of possible insulators the person was wearing because this coud inhibit the current from reaching ground. Thirdly, I would like to say that since the lightning would take the quickest path to ground that a person would gain cover from someone between them and the ligthning bolt. The area of effect would have to be determined by how many volts are traveling through the electrical bolt of lightning at the time of impact. High voltage would definitly arc from one person to another with distance being a factor. Medium to low voltage is just going to hit the first person, and head straight to ground. Depending on what possible insulators the person was wearing would determine how much damage the person took before the electrical current left his/her body. Blah, blah, blah, blah, blah...
Simply, you can not use real world physics for the likes of this spell effect. Mainly because that all current knowledge of how electricity works is nothing but a theory. Yeah, that is right, we are powering our houses, automobiles, computers, and many other things off of a theory. So, please, lets not travel down the path of real world physics. All it will do is give many people a head ache.
Thank you for your time.
EDIT: If you don't belive me about the theory of electricity then ask any certified electrician, and they will tell you the same thing that I have told you.