ShiftDeck's employee app is Android today. Each invite email includes a download link, or use shiftdeck.tech/downloads/shiftdeck.apk on the employee's phone.
On the phone: open Settings → Security → enable Install unknown apps for Chrome (or your browser), tap the APK, and confirm install. Then open ShiftDeck and sign in with the invite credentials.
The current build is signed for sideload (not Play Store yet). If install is blocked, make sure the download finished completely (~64 MB) and retry on Wi‑Fi.