There are many that would state that the tanking role doesn’t really exist in the game as there isn’t really any class in the game that can stand toe to toe with those champion packs in Inferno mode without dropping like a fly but tanking is more than just being able to take punishment.
Tanking is about control.
When you’re talking about control, the monk is a wonderful choice in a group setting.
For an example of a build, start with Fists of Thunder with the Lightning Flash rune for that extra bit of survivability when you’re dancing in and out of close quarters with the mobs.
Another near mandatory tanking skill would be Cyclone Strike with the Wall of Wind rune. This is going to be your bread and butter when it comes to saving your teammates and controlling the mobs. So long as the demons aren’t chasing your high damage dealing party, they can focus on what they do best; burning everything down as quickly as possible.
Breath of Heaven with Penitent Flame rune is also a good option for control with the group heal combined with the AOE fear to give everyone some breathing room.
A relatively underused skill is Inner Sanctuary which can be incredibly useful for giving your teammates a little safe corner to call their own, especially useful when combined with the Consecration rune for the increased duration. Placed out in the open, your teammates can find it somewhat useful to kite around it in a circle but to be most effective, you can split off part of a champion pack in a doorway or a narrow choke point to let your group a smaller, more manageable chunk to deal with.
For mantras, I tend to lean more toward Mantra of Evasion than anything else. The biggest problem is that in Inferno, any champion that hits your allies is probably going to one shot them so the extra group boost to dodge is probably going to save more lives than the tiny shield/HP per second from Mantra of Healing or any other mantra you have at your disposal.
A couple other useful skills that you might think of taking is Seven Sided Strike with Pandemonium for that long stun that can save lives, including your own if you happen to need that few seconds of invulnerability or Serenity which will grant you another few precious seconds of invulnerability for those sticky situations when your reaction speed is all that stands between life and death.
For passives, you’re generally going to want to go with One with Everything for the cheap resists, Seize the Initiative for the double stacking of Dex into armor value and the last slot usually taken by Transcendence to supply you with a steady (if somewhat small) healing.
For gearing, you want to boost that survivability as high as possible so you’re going to want to pick one type of resistance and find gear with that resist, + resist to all, dex and vitality.
If you can find a piece of gear with all of that, that’s going to be your mecca and ideally, you’re going to have that on just about every gear slot. Remember too that if you’re feeling a little spirit starved, it’s perfectly fine to supplement that with a few pieces of gear like a weapon or a monk helmet with some spirit regeneration to help fuel your life-saving endeavors.