Hound is coming soon to the App Store and Google Play as Hound: Fitness with Friends. Until then, the beta is free on iPhone and Android.
TestFlight is Apple's free app for beta apps. The link below offers it first if you don't have it.
Open this page on your iPhone, tap the button, then tap Accept and Install in TestFlight.
Two ways to get it. Google Play is the easy one and updates itself, but it only works if Steve has added your Google account to the test group. Anyone can install the APK directly; your phone may ask you to allow installs from this source the first time.
On Google Play, tap Become a tester first, then Download it on Google Play. If it says you aren't eligible or the app isn't found, you aren't in the test group yet: ask Steve to add the Gmail address you use on your phone, or use the APK.