If you have programmed an Arduino and set up a LAMP server before, go straight to Github for the files and start tweaking.
If you need a little help putting this together for yourself, I'll be adding build instructions for each component as I have time.
I bought a lot of 10 nRF24L01+ modules for about $15, and found Arduino Pro Mini boards at $3.50 a piece in bulk so you can easily build out a dozen beacons for under $100. That'll make a good game around here at the Beach.
With a little modification to the App, you can tailor this to any event.