@gilesgoat “fellow” in this case is not the meaning as a noun (like in “he’s a handsome fellow!”).
It’s the adjective meaning, for “someone sharing an activity or trait with the first person”. Commonly used in the phrase “fellow travellers” meaning “other people who are also travelling”. It’s a way of adding politeness by saying “I’m including myself in the group”.
And in English, adjectives are not pluralised to match their nouns unlike many languages (“blue herons”, not “blues herons”).

Or in other words, the author of the message is also a toilet user.