First rule of playing Master Yi in group fights. You are ALWAYS Last one in & First one out.

Master Yi is extremely powerful but very squishy. In my opinion he's one of the best mid/pushers/farmers in the game. Just remember he does a lot of damage to minions/towers/champions but can't take a lot of damage himself.

If you see red buff up get it asap makes ganking a lot easier.

Pros

Great auto-attack damage
Very fast melee speed
Top of the line carry/farmer/pusher
Great Middle Lane champion
Can run away really well
Once you get good with Yi your Ultimate will always be up (It refreshes if you kill someone while its up.)

Cons

Focused on by Enemies
Squishy early game
Not much Health
When Ultimate is down you can't run away easily
Stuns/Blinds basically = death

Exhaust - This is a must for all Yi's out there. It will help you in 1v1 fights as well as help you gank & flee. Helps a lot with mages as well with recent update.

Ghost - Personally due to the recent changes to flash I strongly believe this is a way better spell for Yi. You can pop it to run in kill quickly while still being able to turret dive effectively.

Other Options

Flash - Some people prefer this over Ghost. Great for turret diving as well as fleeing.

Cleanse - This can be used very effectively if you are being targeted often. I use this when a lot of blind/stun champions are free. Otherwise I would rather buy Quicksilver Sash.

Ignite - Great for First blood & early/mid game. After that it's almost pointless to have this spell. If you think you can get at least 2 kills from it definitely worth it IMO.

Teleport - Decent if you are mid. If you have trouble staying mid without having to go back then take this. It's always nice to know mid is always protected because you have teleport up. Also great for ganking early because you can run to bot/top and gank then teleport back to mid to secure tower.

Clarity - If you think you can't control mana you can take this. I usually just farm red/blue buff so I don't need this ever and would rather have other spells instead.

Rest of the spells are basically pointless for Yi.

This is how I purchase/sell my items.

Boots of Speed
Health Pot x2
Mana Pot x1
Berzerker Greaves
Recurve Bow
Stark's Fervor
Youmuu's Ghostblade
Zeal
Phantom Dancer
Sell Stark's Fervor to buy BF Sword
Infinity Edge

(Quicksilver Sash is a very useful item if going against blind/stun heavy champions.)

At this point you should be face rolling people and you need to decide wheither or not to get armor pen, more damage, or more armor/magic resist.

The last 3 items are completely up to how the game is going.

Alpha Strike --> Wuju Style --> Highlander --> Exhaust if needed.

Alpha Strike --> Wuju Style --> He got help Highlander & Run lol.

Also for 1v1 fights you can

Alpha Strike --> Wuju Style --> Meditate --> Continue to kill them :)