We have deployed the following things successfully at events and have found they can really increase your uptake rates:
Slides at the start of sessions and during breaks that show how to download the app. Using QR Codes for this is a great idea, and we can easily supply.
Provide volunteers with t-shirts that have scannable QR Codes on them or phrases like ‘Ask me how to get the app’.
Put up stickers in bathrooms to allow attendees to scan and download the app.