How to Set up WiFi Access Point to Share Folder of Files for Download?
You can probably do this on your Mac book with an Ad-Hoc network, but I don't know how the performance will be with 20 users so purchasing a separate wifi access point (AP) is probably a good idea (Apple's Airport Express is very portable).  In either case, you will want to setup a web (Apache) and DNS (Bind) server on your laptop.  That is pretty easy to do if you purchase Apple's server application.  (It is possible to do both without purchasing the server, but requires more technical skill.)  Configure the dhcp settings on your AP to give out your computers IP for DNS.  For that to work, you'll also need to configure the AP to always give your computer the same IP address when it connects.  Then, when clients connect to the AP, you can give them your website and they will be able to download files.  

Total cost, approximately $120.

This is really only a high level description.  You may need to ask more specific questions as you get into it and need help.