The members of a corporation are the shareholders, not the employees. The employee equivalent to a corporation is a union, which also has free speech rights.
Certainly not! I'm not in a union and I give money and time to a variety of nonprofit organizations. The EFF, for instance, is a corporation that exists to concentrate the power of many people who care about online rights and freedoms. The Sierra Club is a corporation that concentrates the power of people who care about the environment. Etc.
EFF is just an example. The point is that any group of individuals can form and/or fund a corporation to concentrate their speech on whatever topic they want.
