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.