If you're agoraphobic and are getting stress headaches, it might be time to talk to your psych doc about coping techniques. One thing that I can suggest is to practice with smallish groups of people, and then work your way up to the larger groups.
So other than practice, positive thinking is my other suggestion. Remember that the people in your church group aren't threatening, and that you are in control of the situation. If you need, take a bathroom break during the session, and just go hide in a stall for a couple minutes. I used to do this during school when I got overwhelmed. That and remember to BREATHE. I know it sounds silly, but when we're stressed, we start to breathe more shallowly and that can bring on a headache by itself. So every now and then, take a deep breath when somebody else is speaking, hold it for a couple seconds, and then let it out slowly.
I can totally understand not liking speaking in front of a bunch of people. I get really bad headaches when I'm exposed to a lot of people for any real length of time, although it's been getting better since I've started leaving the house more.
Good luck and keep us posted,'