No matter which method of submission used, the author (in NIH language, the Reviewer) must approve the submission. NIHMS will send an email to the Reviewer confirming the receipt of the manuscript and include a link to the submission for final approval. Manuscripts will not be posted to PMC without final approval from the author.
If your article is published in a journal that submits articles to PubMed Central, the author (Reviewer) must still approve the submission and the final web version of the manuscript. The author will receive an email with links to the submission.
NIH has very good web tutorials for principal investigators, authors, and non-PI submitters.
Both PIs and non-PIs can submit manuscripts. No matter who submits, the author (Reviewer) must approve the submission.