People are commonly misinformed about the meaning of the word 'otaku' while it's actually really simple: "otaku" is a mental illness. It is the unhealthy obsession of an individual. There's several variations of this word of which most are highly degenerated versions:
In Japan, otaku means having an unhealthy obsession with something (which can be anything);
The second degenerated version is that otaku means having an unhealthy obsession for anime;
And last, to most teenagers (and unfortunately most people on the internet), otaku are anime fans (which couldn't be less true).
It's evident that the last form of the word should be disregarded completely. On MyAnimeList, you can use 'otaku' as a word for indicating an unhealthy obsession to anime, as this is an anime community.
As for the spelling, 'otaku' is much like a status, so it's plural is identical to the original form of the word:
Jim is otaku.
Jim and his friends are otaku.
Those otaku need to be put down.
Long story short: If you are an anime fan, you are not otaku. And if you are otaku, you are not necessarily an anime fan.
That concludes the elaboration of this club. Please join this club and help raise awareness.