As a leader of organized trips for a club, we require everyone to sign a waiver. If your group can be construed as "organized" you want to be sure you are not liable for problems, since you are the de-facto leader.

I think there are limits to group size for everyone to have a successful trip. I would be reluctant to have over 20 people, even if there are no regulatory restrictions. You may think about breaking this into two trips of smaller group sizes.

It also helps to have a sign-in and sign-out roster. Everyone signs in with thier name and who to contact in case of an emergency. Then at the end of trip, everyone signs out.

You also need to have an emergency plan. If someone gets hurt, what are you going to do? Who is going out with them. Who is going to be contacted if they really need evacuation. If this is thought out before hand it is a lot less chaotic when stuff goes wrong.