The preposition ó “with” appeared in draft versions of the Aia María prayer from the 1950s, in the phrase i Héru olesse “the Lord is with thee” (VT43/27), along with a chart for this preposition having pronominal suffixes (VT43/29). Prepositional ó is clearly related to the prefix o- “together”. In the third version of the prayer, it was revised to carelye (VT43/27), apparently a variant of the preposition ca as suggested by Wynne, Smith, and Hostetter (VT43/29). In the fourth and final version of the prayer it became aselye (VT43/28), using an otherwise unattested preposition possibly related to √AS “beside”.
In the Quendi and Eldar essay of 1959-60, Tolkien said of primitive WO “together” that it “does not remain in Q as an independent word” (WJ/367). However, the preposition ó “with” reappeared in a phrase from 1969: vá meninye {yó >>} ó le “I won’t come with you” (PE22/162). Based on the revision of {yó >>} ó and the meaning of the related prefixes, it is likely that ó more specifically means “with (one other individual)” as opposed to yó(m) for “with (a group)”. However, I think ó could be used in most cases, and yó(m) is only used when accompanying a group is being emphasized.
Neo-Quenya: It seems likely that ó specifically means “[together] with”, and other senses of “with” would use other prepositions such as lé “with, by (means of)” or the instrumental suffix -nen.
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.