You seem to have a deep misunderstanding of copyright law. Just because you created work as part of a group does not mean you have surrendered your copyright unless you have signed a contract to the contrary.

Without surrendering copyright, you absolutely do have the right to unilaterally license your code in whatever way you want. The entire reason that companies require you to sign IP agreements before starting work is to ensure that you don't run off with the code.

The developers wrote the code and never signed a license to Billy's company. They own it. End of story.