It took a while to really convince Percy that this idea was for the best. But the deciding factor was when Valerie told him, after taking him into the next room to talk in private, about the impending future.
She left out his role in that future, because she was still doubtful that it would even come true with Future boys track record and also because she was sure it would just upset him and cause even more unnecessary issues.
But other than that, she made sure to tell him every detail of the monsters that would soon overrun the earth and that they would need to have a considerable fighting force if they were going to fight back against it.
Besides, they couldn't just leave a secret organisation like this unsupervised after discovering that they existed. Who knows what type of secrets stashed away, or how they could benefit from this organisation in the long run.
