OK, let us begin with the start....
Spearmen A charges into battle...because he is the first he will get only the charge bonus (+1/+1).
Spearmen B joins him...he will get the charge bonus as well and another (+1/+1) because there is already a buddy in cc and another one 'cause he has the SA Gruop Attack...so the total bonus will be (+3/+3) [charge+buddy+groupattack]
The target isn't already dead so Spearmen C enters the stage, he will have a total bonus of (+5/+5) [charge+2x(buddy+groupattack)]
The next one would have (+7/+7) and so on and on and on....
just my two cents....
Anomander Rake