What’s not clear to me is why a corporation’s specific governance would matter. When a company “speaks”, it’s still an individual ultimately expressing themselves, just with the benefit of compensation.
The compelling argument is that it should be quite obvious that companies do not speak for all of their employees, cause most of them are - to what degree can be argued until the end of time - basically coerced to express the "opinion" of the company. At most it's the opinion of the owners, artificially amplified.
And to the question why the owners should not be able to express themselves: They can express themselves individually all they want, but if they want to use a company to do it they should also be personally responsible for everything the company does. And since one of the primary reasons to have a company is to isolate the owners from being sued personally ... there's a bit of a problem here.
But that’s not the claim. A “company” doesn’t speak, individuals do. Just because the individuals do as a result of compensation, doesn’t mean they lose their right to free expression.
You’re focusing on the “group” aspect of this, but that’s irrelevant to the argument. A PR spokesperson has individual rights, even if they choose to use that voice to advance a company’s goals.
Presumably the board could stop corporate speech the majority does not agree with. It would be interesting to re-litigate this case with a company like Facebook where Zuckerberg has special voting shares. If donating to a PAC is just a group of people exercising their free speech, can you delegate your right to free speech to someone else?
Publicly traded corporations are democratic organizations. You get one vote per share you hold. If you want something to happen, the shareholders can vote for it.