if you’re asking about bulk uploading observations to a traditional project, there is not a user interface to do this as far as i know. (i assume this has to do with the potential for projects to be set up with required observation fields.)
that said, it is possible to use the API to effectively add a bunch of observations a project (as long as their observation fields meet project requirements and as long as the observers allow you to add their observations to projects). see https://forum.inaturalist.org/t/bulk-selection-of-observations-to-add-to-a-project/1747/70 and subsequent posts in that thread.