Can I get a list of species that haven't been seen in a place by a group of people?

I think you would pull up the place in explore and add this to the end of the url, appending the project name:
&not_in_project=

From the master list of strings for url manipulation:
https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-2-of-2/18792