If you have added a reservation on your Web Control Panel and it is time to scan the guest's IDs, the steps are very similar to those on a normal Check-in.
Open the Check-in Scan app on your mobile phone or tablet and click "Login"
Enter your Email and password and once on the main screen, click on "Check-ins Calendar"
Choose the reservation in which you want to add guests and follow the Check-in process as you have been doing so far :)
If you need to add more guests to an existing reservation:
The steps are the same: Select the reservation in which you want to add more guests, click on "Continue Check-in" click on "Add another guest" and continue the Check-in process as always.