Under U.S. patent law, inventorship (who thought of a claimed invention) must be determined on a person-by-person basis. For this reason, you must select one particular inventor to indicate to whom this particular inventorship evaluation applies. In regard to using Patent Partner, you may also first add an inventor to the list of inventors for this invention and then select that inventor.
In the U.S., an inventor must be a natural (flesh and blood) person, although he/she can be deceased or legally incapacitated when a patent is applied for on the invention. An inventor cannot be a corporation, partnership, university, etcetera. The actual inventor(s) must authorize the filing of a U.S. patent application.
The question of inventorship is separate from the question of ownership of an invention. Whereas inventorship is solely a factual issue, ownership is a contractual issue. It is typically the case that inventions are owned by other than their inventor(s), as most employment agreements call for ownership of inventions to vest in the employer.