The primary intent of this thread was to develop a means of identifying authors of iNaturalist observations as they are represented within GBIF occurrences data. This identity might be some name or other unique identifier of a person. The replies to the original post and some of my experimentation with the Quercus velutina download data suggest that there are multiple challenges with this. Going with the specific iNaturalist Research-grade Observations might be the most realistic path to take.
A broader and potentially more useful redefinition of the stated task would be to find a means to identify authors of occurrences, in general, within GBIF occurrences data, which would include data that arrives through all institutions that are represented here. For instance, some person might wish to retrieve all GBIF data that represents observations that they performed. See the discussion GBIF Community Forum: My first PyGbif script.
As already noted, there are multiple wrinkles with the broadened definition of the original task. These include the fact that the name of a person is not necessarily unique. A combination of an institutionCode and the username of a person from that institution would presumably be unique, but as we have observed, that username is not necessarily represented within the downloaded GBIF data.