Janna's Monsoon is not that big of a heal, and if used poorly it can be a bad thing. How should it be used in team fights?
It depends who you are playing against, but it's mostly meant to be used defensively.
One of the primary uses is to blow away enemies that jump onto your carries, basically granting your team a fight reset with (ideally) the enemies' initiates being down.
The other main use is to rescue allies who get caught. As Janna, you usually have more movespeed than most, and a damage absorbing shield, so can blast the ult and then quickly escape yourself.
A more offensive use, which requires more skill to pull off, involves jumping into the middle of enemies (usually flash, but a good thresh lantern pull can work really well), and blasting a squishy target back into the middle of your team to be shredded. This is often fatal, so is best suited to removing the threat of a very fed carry.
A less dangerous version of this one often presents itself in-lane as a support, right around when you hit six. A Howling Gale from the bush followed by a Zephyr can get you in a good position to Monsoon an enemy back into your ADC, or burn Flash to escape.
There are also situational uses, like disrupting the ults of Warwick, Kat, etc., or blasting a Crowstorming Fiddle out of your team.
Incomplete list of good abilities to defend against:
Think of the supports first and second priority, to protect the AD carry then to protect the rest of the team. Now the main priority of an enemy team is to kill the AD carry unless the AP carry is fed (but not gonna go into the technicalities). Janna is very effective at protecting the AD carry as she provides a movement speed boost, a shield, a knock-up and a push (her ult).
Using these effectively will always ensure the survival of any competant AD carry. You shouldn't even really need your ult to protect your AD carry if your other skills are used properly. My favorite use of Janna's ult is right after an enemy initiates a fight usually from an enemy tank jumping into the middle of your team with some sort of CC. Now if you are in front of you carries like you should be at all times and this happens this will give you the perfect opportunity to push away any enemy carries about to focus your teams carries. This should give your team time to organize themselves and effectively focus down the enemy.
Basicaly, it is an all around knock back. Mostly meant to be used defensively. However, the possibilities of play are only limited by your imagination.
In adition, what you should not do is:
Hope it helped!