What abilities can I use to protect my team from a Junkrat ultimate?

Lúcio's Sound Barrier, Mei's Ice Wall, and Zarya's Projected Barrier can be used to protect other teammates. It's not quite the same thing, but judicious use of Mercy's Guardian Angel (which can target nearby corpses) and Rez will also counter the RIP-Tire (but not prevent it from killing people in the first place).

Reinhardt's Barrier Field and Winston's Barrier Projector will protect anyone on the appropriate side. However, the RIP-Tire can move through these shields, so they are not as reliable as the previous methods. In particular, Reinhardt's Barrier Field is a poor choice, since the enemy Junkrat merely has to hold off detonating it until it's literally on top of your Reinhardt to guarantee a kill on him.

The easiest, most readily available way of completely nullifying a RIP-tire is Zarya's barriers. Even though it can only protect two players - Zarya herself and a teammate - it is guaranteed to save both of them if the detonation goes off whilst protected by the bubbles. Zarya's barries can absorb up to 200 damage, but will always fully absorb at least one hit, even powerful ones like the RIP-tire's 600 damage and Self-Destruct's 1000 damage. Of course, this is limited to only two players, but "baiting" an early explosion by running out in the open and shielding at the last moment is sometimes enough to waste a RIP-tire completely.

Lucio's Sound Barrier has to be perfectly timed if your team is somewhat damaged, but it can protect the entire team, and it's enough to save even a Tracer caught point-blank.

For the more unreliable method, Mei can step in front of her team, use her Wall to separate herself from the others, again baiting Junkrat into exploding prematurely, and save herself with her Cryo-Stasis. Using the Wall to completely block off a RIP-tire sounds like a good idea in theory, but the tire can simply climb up the wall, look for other entry points, or even wait out the duration. The RIP-tire has a lifetime of 10 seconds, which is usually enough for a wall to vanish if the tire was deployed from nearby.

Though as always, the best counter to a 5-man wipe is a 5-man Resurrect ;)

Considering Rip-Tire does 600 damage in a radial blast, the only ability that can really protect your team is Lucio's Sound Barrier, however...:

• Sound Barrier only provides 500 shields which decays at 100 shield per second, which means...
• It has to be activated, and applied to teammates in the vicinity just as the enemy Junkrat chooses to detonate the Rip-Tire. All in all an unlikely scenario. This is the only situation where low health heroes like Tracer and Zenyatta can survive a point-blank blast.

Consider this as well, Roadhog's Hook:

• Roadhog's Hook can ACTUALLY hook the Rip-Tire in, shockingly enough.
  • This stuns the Rip-Tire for a short amount of time, but enough time to fully kill it at point blank if you time it right
• You can then follow up the hook with your usual shotgun blast that a Roadhog would normally do to other players
• Doing this quick enough and catching the enemy Junkrat off guard can either have you sacrifice yourself for the team or actually get rid of the rip-tire.

Some other considerations that won't necessarily protect your team but may stall the Rip-Tire (as it has a timer before it harmlessly detonates):

• Reinhardt's Shield and Winston's Barrier can block the damage from the explosion ONLY if it detonates outside of the shield; however the tire can pass through harmlessly through shields so this must be timed carefully.
• Using Mei's Ice Wall to force it to either find an alternative path or climb up the wall, taking a few seconds off the timer
• Using Lucio's Speed Boost to get teammates to run away while you still can.
• Take one for the team! This sometimes works; where I know my team is just a little bit behind me. I rush forward towards the Rip Tire trying to attack is so the Junkrat detonates it prematurely, killing me, but not killing the rest of my team.