@fffffffff’s link has good general advice to use a GBIF download if possible. This will automatically include observations with licenses compatible with sharing data ( CC0, CC BY, or CC-BY NC licenses) and having the DOI will take care of citations.
You could also use a download directly from iNat with customized filters (https://www.inaturalist.org/observations/export) but will need to make customized citations based on the observations that you use and their licenses.
You could use a broad GBIF search and then narrow down the observations from it to only those in your project if needed.
If there are observations that aren’t exported to GBIF, you could contact those users individually, and would likely need to cite them individually as well.
On a side note, it is likely that the applicability of data licenses varies by country. In some countries, copyrighting/licensing data/facts probably doesn’t matter - the user can claim it, but it doesn’t mean that the license would actually be held as valid (not a lawyer, just the best of my understanding). Regardless, it would be good form to contact the users anyway.