plural suffix
們 centers on marking a group of people: a plural suffix attached to pronouns or nouns referring to persons.
們 combines the person radical 亻 with 門, which likely contributes the sound. The character was developed to mark the plural of personal pronouns and nouns.
A person 亻 standing beside a gate 門 suggests many people gathered at the entrance, turning a single person into a plural group.
For モン, picture a monk at the gate addressing a crowd: monk -> モン, and the monk speaks to the many people gathered.