I'm part of a group iMessage. One of the people has an iPad and that uses his email, while his iPhone uses his phone number. I've tried telling him to change his iPad to use his phone number too, but no one else in the group message apparently experiences this. On my iPhone, I end up with two separate group conversations, depending on which device he used (or rather, whether he used his phone number or email address). Even if I delete both conversations and let new ones start, there'll be two. However, on my Mac using the Messages app, it always retains it as one conversation.