We usually only bother with complications if the group is considered really close together, where a sort of cover is provided by each other. Any modifiers below are in addition to the contextual modifiers for range, specialist skills, cover, moving targets and so forth.
Firing Into A Group, Without Target
We use some methods as described in Apocrypha Now, WFRP1 Supplement.
When firing into group of creatures (foe or mixed friend and foe), without declaring a target, the shooter gets (BS)+5 for each enemy considered to be in that group (including friends). If they hit, assign each group member and roll to randomly select the target that is hit.
Firing Into A Group, With Target
In some cases to keep things unrealistic but fluid, we have simply invoked a (BS) -20 modifier for the extra difficulty, and if the player misses, then don't bother with seeing if it hits another target. Sometimes simple is better. If players do want the chance of friendly fire, then we have used some rules found in a mixture of official literature, and some Warpstone magazine articles.
When firing into a group (foe, or mixed) and declaring a target, the normal (BS) -20 modifier applies. If you miss, and the score is between the modded BS value and your natural BS value, then you have hit another combatant. GM applies a numerical value to other combatants and rolls dice to determine the unfortunate victim!
These notes have been added to the
House Rules Compendium