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:
- your own website
- the iNaturalist home page (https://www.inaturalist.org)
- the iNaturalist app page in the Apple app store (https://apps.apple.com/us/app/inaturalist/id421397028)
- the page in the Google app store (https://play.google.com/store/apps/details?id=org.inaturalist.android)
- the project page in iNaturalist (https://www.inaturalist.org/projects/{your_project})
- the link to joins the project (https://www.inaturalist.org/projects/{your_project}/join)
- a journal post in the project in iNaturalist (https://www.inaturalist.org/projects/{your_project}/journal/{post})
- something else
you can make your own QR code using many different tools. if you have no ideas where to even begin, here’s what i would use to encode a text string such as a URL: