Freedom of association. You are free to invest or not invest in a corporation. You can be a customer or not. You can be an employee or not.
The idea that a group of people (organized as a church, club, corporation, partnership, union, political party, non-profit, etc.) can only 'speak' as a group by getting every 'member' to explicitly sign a document for each and every instance of 'speech' or expenditure of money is entirely unworkable.
Freedom of association is one thing. Making use of the legal privileges attached to "corporations" and "nonprofits" for uses way beyond the intent of those privileges is quite another. Yes, no large organization can get the explicit assent of every member to every official statement it makes. But that doesn't mean corporate management has carte blanche to do whatever they want with company money.
But, as I posted in another response in this thread, the real fix for this is legislative, to change the laws governing corporations and nonprofits to fix the huge abuses of corporate governance that go on today.
The legal privileges that corporations get that individuals don't get, like being able to deduct operating expenses from their taxable income, and the legal privileges that individuals can get by forming a corporation, like being shielded from personal liability. Corporations are not the same as individuals, but saying "corporations have free speech rights" as though it were an obvious fact assumes that they are. Corporations are legal constructs, and it's up to us what rights, if any, to give them.
The idea that a group of people (organized as a church, club, corporation, partnership, union, political party, non-profit, etc.) can only 'speak' as a group by getting every 'member' to explicitly sign a document for each and every instance of 'speech' or expenditure of money is entirely unworkable.