Path of exile - party tanker build

You are better off not as a tank but rather a support through curse and aura use.

Curses go a long way to reducing boss damage output. Combine enfeeble with temporal chains for example. You can also add in other damage boosting curses like elemental weakness and vulnerability.

Using auras like determination, discipline and grace you can boost the party members defenses. Add in Heralds that party members aren't already using and get all of the aura mana reserved reductions and effect/radius boosts to increase these effects.

Some unique items are tailored towards being a supportive character. The Victario's items come to mind. Victario's Influence boosts auras as well as significantly decreasing their reservation cost. Arguably the most useful unique for a support character. Victario's Charity Allows you to generate charges for allies and boosts aura radius. Lochtonial Caress generates charges and gives you Conduit which shares charges with party members.

Since these items and other abilities usually gives charges on kill you can link a damaging skill to culling strike to get kills without needing much damage. Of course it won't work when your party members are 1-shotting so you are usually better off focusing on cursing and auras.

I would suggest trying an Empire's Grasp reverse knockback build to drag enemies where you want them and hold them there instead of taunting. This will at least have the effect of keeping your ranged party members safe from their melee attacks.

Some builds to try that can include reverse knockback are dual shockwave totems, RAT nuke, molten strike Mjölner, or a fast attacking cyclone build.

Combining reverse knockback with auras, curses, vigilant strike with The Vigil jewel, and ascendencies which can affect your allies such as Guardian, Champion, or Necromancer can let you fulfil a tanky support role.

Guardian's abilities are especially well suited to buffing party members defensively, whereas Necromancer's are a huge boost to offense, even after the recent nerf.

You can generally taunt bosses. Only the Shaper and his guardians are immune to taunt. It may not stop the bosses hitting your team mates at all but they will take less 10% damage when attacked by an enemy that you've taunted. Make sure they're not taunting enemies themselves which they might be doing if they are using any war cries.

Probably the best way to protect your team mates is to use the defensive auras (grace, determination, discipline and purities) and focus on improving aura effectiveness as well as aura area. It is possible to use all of these if you get some passive skills and items that reduce mana reservation. Focusing your own character around energy shield also allows you to use some auras by reserving life using blood magic (particularly with the Prism Guardian unique shield). This will ensure that all of your team mates can tank the boss and deal lots of damage at the same time.

The ascendancy classes Necromancer and Guardian reward for granting auras to your allies. Guardian in particular is good for a defensive support character as it can also grant a lot of bonus energy shield and damage reduction. Champion gives extra bonuses for taunting.

It would also be effective to focus on cursing enemies. Bosses take less penalties from curses but you can increase curse effectiveness with some passive skills. You can also get a passive skill and some items that allow you to apply more than one curse to an enemy. A highly effective temporal chains and enfeeble will massively weaken a boss.

If you get endurance charges then you could get the Conduit keystone passive which will share your endurance charges with enemies.

Another option to stop bosses would be to focus on making hits that stun enemies. Look for things that give increased stun duration and, particularly, enemy stun threshold reduction. Maces are probably best for this as they have implicit properties that improve stun and lots of mace passive skills also help. It's fairly easy to get some good stuns on bosses playing solo, but it may be difficult with a team unless you really go all out.

Another thing you could try would be to chill bosses. Chill has a base duration of 2s which can be improved with increased chill duration. Increased chill and ailment effectiveness will make the chill slow the subject more. Like stun, you would probably need to really focus your character on this. There are also some abilities that let you maim enemies, slowing them even further.