To get the iOS version of the app, follow this link: https://testflight.apple.com/join/3rOzgkjA (note, you must be on your iPhone or iPad when clicking this link)
It will have you install an app called TestFlight. Using TestFlight, you will then be able to get the Groups.io app.
All that said, I'd like to get people to start using it so we can find any bugs and get feedback. Please post any feedback or bugs in separate topics. Only reply to this topic if you have a problem getting the app.