Super Smash Bros Ultimate is a great multiplayer title, but you may have missed the return of the "Stock Share".
Allowing you to share your remaining stock (lives) with a fallen teammate, it can be key to turning around a losing battle.
Super Smash Bros Ultimate: How To Share Stock
Stock sharing is triggered by the downed player, so try not to upset your teammate by pinching one at the wrong moment!
If you've been downed and your ally has more than one life available, press A+B together to take one of them and get back in the fight.
Doing so can help you get the drop on your opponents. This can be used if you have a series of teammates, too, with the stock being taken from the ally with the most available lives.