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.