VCA groups utilize Voltage Controlled Amplifiers to maintain the relative volume of instruments in a mix. You can assign channels to a VCA group in the same way you would as assigning them in a mute group.
The VCA channel does not have any signal sent to it. You are simply choosing which channels will be affected by the amplifier. Whichever channels you assign to the VCA group will have its fader volume controlled by that groups amplifier.
The benefit of a VCA over a regular group send or bus is that if you turn down your group in a VCA your post fader sends are proportionally turned down so FX will maintain their wet to dry ratios. Also you do not have to route the channels to a subgroup which leaves more routing options available for you to work with..