Savage Worlds has very little to say on the subject of firing into melee. The only rule is the Innocent Bystander rule - if you roll a 1 (or a 1 or 2 when firing full-auto), you hit someone other than the target. With Bennies in play, this doesn't happen very often, which means that firing into melee becomes rather trivial. This can have some negative effects on game balance, to put it mildly - shooting characters can hang back while one character tanks and just pick off the combatants nearby.
For this reason, I've imposed a penalty for shooting into melee based on the other people nearby effectively providing cover. In general, I call this medium cover (-2), to reflect both the possibility of a large part of the target being obscured and the chaotic situation in the melee. This makes the possibility of actually hitting the target significantly lower, and rebalances the tradeoff between melee and ranged in most cases.