Your guests will receive confirmation email with accommodation voucher after they complete their booking. Confirmation contains information about possibility of using My Alfred web app.
Guest gets brief list of app functions like online check-in, ordering of services, payment options, information about hotel, etc. The most important is unique six characters long reservation code for the administration of their reservation.
Guests can fill in their unique reservation code in the app and login to their reservation administration. They can also click throught the confirmation email/accommodation voucher which will log them in directly if they have the app already installed.
You can also remind them about the app by your own mailing campaign where you can you special placeholder with reservation code.