Renting out a spare room is capitalist. It doesn’t matter if it is one room or a room in a hundred houses, both are by definition and inherently capitalist. Which makes every single person renting out a listing on AirBnB “a capitalist” unless the price to the non-lister is exactly zero.
Sharing is the term used for when you don’t expect or receive any compensation for the use of your capital (time, property, etc.). The moment you ask for anything in return you’ve crossed over from “sharing” to “capitalism”.