Why does saint augustine need the "gang" during his "theft of the pears"?

Answer 1

According to his account, he would never have done it were it not for his peers (he says this about a zillion times). He didn't want the pears themselves... the only sweetness he claims to have found (even from the pears) was the sweetness of sin. This is probably why he carries on about it so - the act was evil simply for its own sake and not for any personal gain at all.

As to why, the most telling line, I think, is this one:

"...but when it is said, 'Let's go, let's do it,' we are ashamed not to be shameless."

His motivation, then, might be the acceptance of his peers and enough pride not to want to appear less to them. And that is why they had to be along - without them to perform and demonstrate for, there is no need to make the performance.

That's my take, anyway. Hope that helps!