Barret - "Cosmo Canyon... this is where AVALANCHE was born... I promised my guys someday... when we saved the planet from the Shinra, that we'd all go to Cosmo Canyon and celebrate... Biggs... Wedge... Jessie... Now they're all gone... died for the planet."
In the original game this clearly hints that Barret created AVALANCHE and that he, Biggs, Wedge and Jessie were the main men behind it. There might very well have been minor AVALANCHE supporters along the way, not any of big importance though - like the ones in BC - because they were never presented or even mentioned. Like Deepground and SL this concept might be squeezed into the story if you really want them to, but that doesn't mean they're not retcon.

The expanded AVALANCHE wasn't even mentioned in the original game and that would've been an important part of the storyline don't you think? Before Crisis has just been added later on, not because there was something left unfinished in the original, but because SE saw a potential profit source and thus expanded the story with unnecessarily retcon.