INaturalistIOS is the official iOS app for submitting data to iNaturalist.org.
We're using a number of submodules so there's a little more than cloning:
git clone git@github.com:inaturalist/INaturalistIOS.git
cd INaturalistIOS/
cp config.h.example config.h # and edit to configure for your project
git submodule init
git submodule update
cd Vendor/Facebook && ./scripts/build_framework.sh
That should get you set up for local development with the Simulator. If you want to test on actual devices you'll need to get a provisioning profile from Apple and configure the project to use it: https://developer.apple.com/ios/manage/overview/index.action.