You'll need FileMaker Go dowloaded for Free onto the iPhones and iPads.
Some preliminary info that is gradually drifting out of date, but which has some useful info: An Old Dog learns some new tricks...
This comes from when I first started designing for FM GO.
Synching the data back to a central file may require a 3rd party synch tool. (there is a forum just for FM Go here that you will want to visit and where you can research this option more.)
Here is FileMaker Inc's document on synching: http://www.filemaker.com/solutions/ios/docs/fmi_guide_sync_en.pdf