So, here’s the way we do it for now. We’ll see how it turns out.
Unless the shooter has the Precise Shooting proficiency, aiming across melee or at someone engaged in melee may hit any of the participants along the line of fire.
Roll to a cumulative -4 to hit for each participant, starting from the one closest to the shooter. Same applies when shooting across a melee, unless elevation provides a clear line of sight.
Eg. when trying to shoot at the orc fighting your friend while standing some way behind your friend, first roll at -4 to hit your friend, then at -8 to hit the orc. If standing behind the orc, first roll at -4 to hit it, and on miss at -8 to hit your friend.