If the hunter is BM spec, the hunter pet will be bar none the best tank in that group. It isn't even close. If you're interested in an analysis of lock pets vs hunter pets, I wrote one up a while back as I was curious as well. It's very informal and unscientific but it does give you a good feel for how they fare against each other, from which you can infer how well they'll do against mobs.

http://vnboards.ign.com/wow_hunter_c.../100234784/p1/

That was post-2.0 patch and some minor things have changed since then. Really, though, if it's instance pet tanks you want, you can't beat hunter pets for survivability and aggro on any single target.

Now, you specifically mentioned wanting to be able to tank multiple mobs with the pet. The only one that can do this satisfactorily is the voidwalker. Hunter bird/bat pets used to be able to but they nerfed the AoE aggro of screech where it's almost useless when fighting 2+ targets. You will need CC once you hit outlands unless you seriously overlevel the instance or any pet will get chewed up much faster than a shadow priest can keep it healed.