In Smash Ultimate (just like all other Smash games) you have only one save file, so you can't just load another if somebody else wants to play without messing with their progression. I've tried looking into this on the Nintendo website, but that has not been very helpful. I have heard that a function like this exists for certain other games who function like this (Breath of the Wild), but I can't find anything about this for Smash Ultimate.
Note that I am not asking about the Nintendo Account that is used to shop for games and download DLC, I'm asking about the user account that you make when first starting a new Switch.