Well, if I go through an example, might that help?
Myself, the Freeguild player has a unit of 20 Freeguild Handgunners, a unit of 20 Greatswords and a unit of 10 Freeguild Guard with swords and shields.
I've move the 10 swordsmen up front in my turn, and the handgunners are a little behind and to the right a bit, where at least a couple of them are within 5" of the swordsmen. Likewise, the halberdiers are 5" back and to the left.
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My opponent has his turn, and in the charge phase charges with his Blooodreavers, and successfully ends up within 1/2" of my Swordsmen, but the Bloodbound are still 6" away from both the Greatswords and Handgunners
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The Great Company rules say that at the end of each enemy charge phase a great company unit can support another great company unit. As long as the unit doing the support is more than 3" away from the enemy - so the Handgunners and Greatswords qualify in this example. Also, that the unit they are supporting is within 3" of the enemy - so the Swordsmen qualify, and that the units doing the support are within 6". As we've said already the handgunners and greatswords were 5" away from the swordsmen, so they can both act!
The act can either be a shoot as if it was the shooting phase, or a charge as if it were the charge phase. So, the handgunners could shoot, it doesn't actually specifiy they have to target any unit that is within 3" of the unit they are "supporting", so they could shoot anywhere, though they might want to help out their buddies. The greatswords could charge, and again, doesn't have to charge the bloodbound, but it could be the only unit in range to charge.
So, supposing the greatswords charge the bloodbound (likey to succeed as they are 6" away) and the handgunners then shoot, killing a few of the bloodbound.
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The bloodbound with a smaller unit is now facing two enemy units, rather than one.
Supposing both sides roll terribly, and nothing much happens in the combat phase, and battleshock phase. Then my opponent wins the roll-off for the next turn, and his hero phase, movement phase, shooting phase don't have any impact on this area of the battlefield, so it's his charge phase. At the end of it, even your opponent didn't charge, you can check to see if the great companies rules are applicable, and in this example, the handgunners would be eligible to support again, so they could charge or shoot.