He was with M80 and from what I understand Sen just thought M80 would win and he would continue with them
As a result, they formed the team without him
If they did know, he would atleast get a trial
They planned on keeping Bang
Bang got an offer from Sen or something and then discussed it with Zander and Asuna and some of his friends and they said he should go if he wants to
There is a clip of Zander explaining the whole thing