Are there any "official" multiplayer rules out there? I'm looking for guidelines for Team (2 vs. 2) and Cutthroat (1 vs. 1 vs. 1 vs. 1) styles.

as of today, none of such rulings exist, you will have to work out one urself XD

Yeah, you need to figure out rules for yourself.

I usually play it as ways like

Cards like Raigeki, Harpie's Feather Duster and such will affect BOTH opposing players (assuming it's 2 vs. 2) along with any card that says "Destroy all <insert card type here> cards on the opponent's side of the field," or effects like Card Destruction, Morphing Jar, Cyber Jar, Fiber Jar, etc, while stuff like Delinquent Duo, Confiscation, Dark Designator and other cards that are meant for just targeting 1 player will only affect 1 opponent of the user's choosing or, in cards like Fissure, Smashing Ground, etc, will only affect the lowers ATK/highest DEF/etc monster on ONE of the two player's fields.

Traps like (if traditional) Mirror Force only work on the player doing the attacking. It won't wipe out the ATK monster's on the teammates field unless the teammate is the one attacking. The same going for Widespread Ruin and other such cards.

You can use DEF position monsters to "block" an attack made by the opponent and only face up DEF monsters (not fair to have it f/d since you can always just move a Cyber Jar in the way)

All players start with 8000 LP or, to make it interesting, you can count both player's LP in 1 big lump so both teams could have 16,000 LP and once that LP hits 0, the team loses and LP will be taken off regardless of which player gets ATKed or affected by cards so if Team A has an open field and Team B has a Hayabusa Knight on the field, if they attack Player A1, the TEAM loses 1000 LP and if they attack Player A2, they lose 1000 more leaving them with 14,000 LP left.

This is optional, but I usually play by saying that, if you don't have LP as one lump, if one player goes down, the other player can STILL continue to duel until he/she loses/forfeits/etc.

Stuff like Exodia, FINAL, Final Countdown, etc will still automatically win the whole duel for the team.

Last Turn will involve ALL the players in its effect of battling and such and, if at least one player from each team still remains, they will continue the duel as normal until one of them wins. So it can end up as 1 on 1, 1 on 2, 2 on 1, etc. If it still ends up with all the player's, like normally, it will end up in a draw.

Yeah, complicated, but it's a fair way to play, IMO.

How we do it in my group is similar. With our rules, though, anything that affects one opponent (like Poison of the Old Man) affects both opponents instead.

Thanks for your imput! :)

It's usually a matter that is subject to all the players in the current multiplayer game. We decide how we're going to play since cutt-throat style is more of a compromise between players. So usually the biggest rule we have is that you can't attack the same player twice in a row to prevent cheap ganging up. Otherwise, mostly any rules are fair game.

From the Yu-Gi-Oh Tournament Policy document:

These are the only sanctionable formats at this time. You are well within your rights to have a multi-player tournament, you just can't sanction it.