INaturalistIOS is the official iOS app for submitting data to iNaturalist.org.
We've just finished transitioning to cocoapods: gem install cocoapods, then pod install.
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.