fundamentally, i think the key to what you’re trying to do might getting a list of project members. see: https://forum.inaturalist.org/t/how-to-export-the-list-of-members-in-a-project/24750/3. that post mentions a tool by @kildor which may help with this; however, i just tried it and it may have problems returning results when you use the page in English instead of Russian, or when your project has a lot of members. you should still be able to get things via the API though.
once you have a list of members, you can just filter by them using a comma-separated-list of user IDs. here’s an example of that: https://www.inaturalist.org/observations?place_id=any&project_id=birds-of-the-world&subview=table&taxon_id=7823&user_id=astra_the_dragon,pisum. it should be noted that that there’s probably an upper limit to how many user IDs you can filter for this way, but it’s probably something like 500+.