algraff is right. In a public place, people have no right to the expectation of privacy. As I have understood it in the past, that includes public Church activities. The only place that a person can expect privacy is at a private residence or private property. Even then, if I am at your private property, you can take a picture of me and I can't do anything about it.
The most important thing to think about, IMHO, is whether the possibility of angering the church members is worth just not going and having people sign off to have the pictures posted. In other settings, that doesn't really matter, but in a church setting, certain attitudes and pleasantries are expected more than in a secular setting.
Hope that helps.