For example, if PRISM is really just an internal NSA tool for interfacing, searching, storing etc information procured from these providers from different means (such as NSLs or FISA warrants), then obviously no company joined it. It's an internal tool! The idea of 'adding' just becomes ensuring normalization of data and other such 'mundane' things.
Note I say, 'for example'. Because amongst all the yelling about, we still don't actually know what the fuck PRISM actually is, and arguments just revolve around how charitable/cynical you are about the parties involved.