Yep. Celebrities are people like anyone else. As long as we live in a representative, democratic system, every member of the public has a right to get involved if they so wish.
In what terms? I don't have an issue with business owners pursuing their political interests, but I'd support harsh limits on political donations, to prevent the wealthy (business-oriented or otherwise) from having more "say" than anyone else. Beyond that point, business owners have as much right as anyone to get involved.
Churches? As long as they are completely apolitical. Have no opinion expressed or supported in favor of any candidate or party, or even political policy. The sole exception I would make would be in protecting their own religious rights, for themselves. By which I mean having the freedom to ask for religious holidays off from work without discrimination, or the right for Jehovah's Witnesses to deny blood transfusions for their own medical treatment. Not anything that affects anyone else, so no pro-life advocacy allowed, no stance on marriage equality, etc. Get into any of that, poof, there goes your tax-free status, permanently. Outside of tax-free status, if you're fine giving that up, and with the same eye to political donation caps above (which should be applied to coordinated collection efforts as well), they should have as much voice as anyone. I just have an issue with demanding a tax-free status while also demanding the right to meddle in politics; that was supposed to be a "you don't bother us, we won't bother you" arrangement.