access control via barcode-smartphone

This is the scenario: in a private party, every guest will be provided with a wristband that has a printed barcode. One unique number (barcode) per guest. On the other side, the host will feed our project with the numbers of the wristbands provided by us. The data input will be done on a desktop computer a few hours or days before the event. On the day of the event, the host will have multiple helpers, each at different access points to the party, and each with an androird or iphone smartphone. Using their smartphone the helpers will scan the barcode on the wristband of each gust to grant access. If that number has been scanned before a red light will show up on the screen or an audio alarm will sound an the guest will not be able to go through. This app is to check people in, not for checking out. Smartphones will be connected to the internet through mobile data from the telephone provider or through an available WiFi. We are the wristband manufacturer and we will provide this app to our customers. We need to be able to provide data support to 5 different customers at the same time.