A preposition appearing as yó(m) “with” in Late Notes on Verb Structure (LVS) written in 1969 (PE22/168). Earlier in the same set of documents, it appeared in a sentence vá meninye {yó >>} ó le “I won’t come with you” (PE22/162), but there it was replaced by ó. These were probably based on primitive forms ✶jō(m) “together (plural)” and ✶wō “together (two things)” from the Quendi and Eldar essay from 1959-60 (WJ/361). If so, Tolkien may have revised {yó >>} ó in the sentence above because “you and I” is a pair rather than a group.

Neo-Quenya: For purposes of Neo-Quenya, I would assume yó meant “[together] with” when applied to groups of things, as opposed to ó for two things together, though I would also assume ó was the default and yó was only used when plurality needed to be emphasized.