Hi Sarah,
I assume that people are coming to this first meeting for information - not to discuss a book?
If so, I suggest you use the meeting to layout exactly what you have in mind for the group - the types of books you want to discuss, how often you plan to meet, what the group's policy will be on people coming to the meeting without having read the book and so on. Look through the book club logistics forum for ideas of things you should be agreeing as a group up front. Normally, I'd suggest that this discussion should build to a consensus among the group members as to how to go forward, but with such a big group there will be way too many voices to open it to debate so you need to clearly lead the meeting and spell out how you see things working - while obviously being open to questions and flexible if a point is well made.
Once people know what you have in mind, there may well be some in the group who feel that it's not going to be for them for any number of reasons, and decide not to sign up for the club after that first meeting. Perhaps some of those people will end up wanting to form their own group? For example, you maybe planning to meet in the evening but there maybe some for whom daytime works best. Others may have signed up thinking that the book club was going to focus entirely on specific types of books such as Christian literature, whereas you might envisage something different.
The chances are that many who put their name on the signup sheet won't even come to that first meeting. But if more than about 20 turn up to this meeting and it looks like they're all really keen, I suggest you tell them that you're going to keep the option open of splitting the group in two if the number of people attending after the first 3-4 meetings is unmanageable, and have some plan in your head of how you might do this. That way, if down the line you find yourself having to do this it shouldn't cause such a rift. Equally, explain that because there are so many people interested, you really want to be sure that those who do join will be committed to it, so put in place some fairly high goalposts about attendance.
I would have said that if you have about 35 express an interest, probably 20-25 will show up to an exploratory meeting. Of these maybe 15-20 will be interested at the end of the meeting and you'll probably lose another 20% or so over the next few months as they realize that their good intention to stick with the book club doesn't work for them.
In short, there's a good chance the group will naturally whittle down to about 15-18 of which there will always be a few who can't come to a particular meeting so you'll probably be fielding a dozen or so at an average meeting in a few months time (but if that's not the case and the group's too large you still have the option to split it later).
I mentioned the regularity of attendance earlier because this is likely to be your biggest hurdle in starting a potentially large group at your church. The larger the group, the greater the tendency people will have to think that if they skip a meeting it won't matter because there will be lots of other people there. Some people also seem to feel less of commitment when meeting in a public place - presumably because they have the perception that there will be plenty of others there and they won't be missed. This is a topic that's been touched on in some of our book club chats http://bookbrowse.com/featured-bookclubs/archives.
