What are the major differences in group-skydiving as opposed to "single" skydiving? Surface area/the cross-sectional area is greatly increased, so my thinking is it should decrease terminal velocity as well as a couple of other factors. But mass changes too (since there would be more people attached), so would that, by any chance, cancel out (for lack of a better term) the large surface area's effects?
 A: (been there, done that)
In general, a large, tight formation will fall slower than a small one. But not a lot slower. Body position (drag) makes a larger difference. I can make a very substantial difference to my fall rate with a small change in waist angle - picture it as laying on a beach ball face down vs. face up.
A single jumper can change their fall rate from about 150km/h to 300km/h simply by rotating from flat to head down and tucking everything in - your goggles start to vibrate at those speeds.

Two skydivers with the same m/A ratio will attain the exact same terminal velocity 

Not quite. Induced drag from body position is a major factor. A 50kg girl can easily pass a 120kg fat boy if she knows how.
A: It depends from the "choreography" the skydivers are following: given the terminal velocity formula, it depends, giving the same air/gravity values only by m/A, so if two or more skydivers have similar masses and areas, we get (nm)/(nA) which gives again m/A. 
Of course the change of the rotation of any skydiver will affect the terminal velocity of the group. 
