How exactly does this thing work? It says it allows ganging up at range, but it's far from clear what this means. Is this just a straight +20 to hit for everything, with the players being considered to outnumber every individual enemy 4-1? If so do they have to have a ranged weapon equipped? Does it have to be able to shoot to the targeted enemy? Do they have to attack the same target to count as ganging up? If so does the first person who fires get the bonus?

Really seems to me like this needed a little clarification in the rules, extrapolating the melee ganging up rules to range is far from an easy matter.

Ganging up works when the attacking group outnumbers the opponent they are engaging.

The key here is engaging. To engage a target in melee, all a character must do is be in a position where he can make an attack and decide he engages. No attacks are necessary to count as being engaged. You can be able to attack a target and still decide not to engage them. BUT you cannot make melee attacks against a target UNLESS you engage them.

THUS

To gain a ranged Ganging Up bonus from the signum, the battle-brothers must ENGAGE the targets with double the number of characters being engaged. Since you MUST be able to attack the target in order to be engaged, you MUST be able to see it and hit it with a ranged weapon, though you aren't actually required to make an attack.

NOTE: You may only engage* as many targets as you can shoot (normally one). IF a character has talents or options allowing him to shoot multiple targets (Independent Targeting or Walking Fire for instance) then the character may choose to engage multiple targets BUT each enemy engaged counts for outnumbering for the whole group.

* No, you cannot get free strikes against enemies engaged by ranged weapons, "Engaging" in this sense only applies due to the signum rules, you cannot normally "engage" targets at range.

Edit: It is also possible to choose not to use the signum, to allow some Battle-Brothers to benefit from the signum without others polluting the link. Say a devastator shooting a horde while the other KT members shoot something else with the signum.

Examples:
A 4 person KT fights a group of 3 Chaos Space Marines and a 10 person horde of cultists.

All 4 members of the KT engage one of the CSM, every KT brother who attacks the CSM gains +20 to hit him from the signum.

Next all 4 target one CSM but the devastator uses walking fire to hit (and engage) the other CSM. This time the KT only gets +10 for outnumbering since it is now 4 KT brothers vs. 2 CSMs.

Finally the KT targets the cultists, but since they are targeting an area target, versus specific entities, AND you do not gain outnumber bonuses against hordes, they would not receive any signum bonuses.