A lot of the focus of this game is on one-on-one. I mean, just look at how tiny this section is. Out of curiosity, though, how many have done a party game setup just like the official Smash games?
What I mean by this is a disorganized, casual match with multiple friends and/or family. A free-for-all.

There's also another question behind it; have you been able to? Can your machine physically handle it without lagging? Have you been able to adjust the controls for four players? Etc.

I've been blessed with three siblings who play way too much Smash like I do. We use two keyboards with controls based around WSAD, arrow keys, FVCB, and /789 (since Delete can't be assigned). There's probably better ways to set up those controls (we don't have controllers, and there's not enough room to assign separate jump buttons or C-sticks), but it runs fine, and we have a lot of fun.

But I imagine that for most players it's difficult to find three other people who a) play Smash, b) are interested in Flash fangames, and c) are willing to gather for a 4-player free-for-all. Especially given that SSF2 players are nerds.

Online matches with four players tend to be so laggy and riddled with disconnects as to be virtually unplayable.

I used to play a lot with my sisters too actually, I still do, its just pretty rare nowadays

I use to play with my brother, but he quit playing in 0.9a
Now I have no friends to play with

I have my cousins that come over once in a while to Smash with, but apart from them I don't really get much opportunity as my friends are busy all the time.