Installing SwiftSole is a two step process. Although SwiftSole is an iOS app, it isn't available on the App Store like other apps, and must be installed through a separate app(which is on the App Store) called TestFlight. Think of TestFlight like a hosting app through which you can launch, install and update SwiftSole.
Install TestFlight on your iOS device. You can download TestFlight here.
Once you have TestFlight installed, check your email for an invite link to SwiftSole. This email should also contain you username and password to login with.
Accept the invitation and click
Install to install SwiftSole.