If you have the iNat app, Android phones will open qr codes for observation links in the app but other links like projects and taxa only open in the browser. I don’t have an iPhone to test. Maybe create a feature request to open project links directly in the app instead of the browser. I think it would be handy.
opening iNaturalist and inviting folks to join a project are two separate things. you could encode one QR code that opens iNat and another that contains a message, but it’s probably easier to make a single QR code of a URL that points to some web page which contains an explanation of your project and links to do various things (download the app, join the project, etc.).
if you want go with the URL approach, you have to decide which URL you want to direct them to:
There’s no “invite link” for iNat Projects. You can encode a link (URL) to your Project as a QR code. Opening the QR code takes them to your Project’s page in a browser. If they’re already logged in to iNat, they’ll see the usual “Join” link.