As others have mentioned, the UI is super cluttered and for some reason there’s this cheesy animation before you even get to the main screen.
Secondly, the features that are personal to you, like group fitness registration, requires the app to pass authentication data to the ARC website, which means you need to enable activity tracking for the app. If you have “Allow apps to request to track” toggled off in your Security settings, the app is automatically denied (this is by design). However, even if you toggle the feature on, the app won’t ever attempt to ask for permission again — it’ll just give you an error message.
So, you basically have to uninstall the app and reinstall it to have it ask you to track your activity so that you can register for group fitness classes on the app and other features that require your authentication data to be passed to their website.