I can't force them. But they won't lose anything they already have- the rules only dictate how allied teams must treat each other, nothing forbids anyone from assisting teams outside of it, as well. They just wouldn't have the extra benefit without meeting the conditions in return.
Ideally, it would be best if everyone would agree, and so everyone would benefit, but.... I don't think some teams can be convinced just yet. At least, not by me. But anyone can join- or leave- later, if they like. And some of the teams might be easier to convince if they see it in action first, I think.