When you think about it, this feature on a church website is a no-brainer:

The example above is from Church of Christ the King, and I first saw this idea on the C3 website.

It's a no-brainer because it's low-hanging fruit. People want to come to church, and this form allows them to tell you that they're coming. You can then use this information to meet them beforehand with a coffee at the ready, show them around church and help introduce them to other people. 

If people are coming this weekend and want to let you know in advance - this is great! Why not make it easy for them? I'm going to work this into the next version of my church's website. 

If you'd like to get something like this set-up and don't know how, Wufoo is a great place to start, and they even have a 50% discounts for churches. 

After Sunday, you might even like to follow-up with a first impressions survey, to learn how you can serve them and future guests better.