Using the right build is important for teamfights! You should use the full Tank build only if you have no other full tanks in your team. If you have 2 AD/AP offtank champions in your team you can build the Support items. If you have one Tank and a AP/AD offtank in your team you can use the AD Offtank build, for better support with your stun autohits. If you have 2 or more Tanks, it's better if you build ADC similar, for more damage in your team.

dependent from build you have to play agressive or defensive:
PO#3: You are the first who engage in Teamfights you try to stay in and fight as long as you can! If necessary you can jump out with your W or flash out of a fight. You have to knock up at least 3 of 5 in a fight. Than your Team should kill them realy fast. A Braum Ultimate can dicide Teamfights, so a good timing is important.
PO#4: You're the support you have to ward a lot and have to Tank much too. You're the one who leads the team. You start to fight, you give safety for a champion who came out with low life and protect him, untill he went back to the base.
PO#5: You let the Tank of your Team engage and you are the 2nd who go in and start to fight! Your team need your extent stunning to kill enemies easier and focus out the enemy carry. And you deal a lot damage and can tank much too.
PO#6: You engage as the last. Try to slow someone of knock up some enemies to go up to them and attack. Than you can stun with your Ravenous Hydra more than one at the same time. You dont deal much critical damage but you have a good attackspeed and strong attack damage, so you can kill the marksman in 1vs1 easy.!

Yasuo, Lucian, Vayne, Jinx and Ezreal