Ariana Grande 's concert on her "Eternal Sunshine" tour is bringing out tons of winners ... because a couple of "Victorious" cast members came out to show their love for the star. The pop star rocked the Kia Forum in Inglewood, California Wednesday night ... and standing in the VIP section were a pair of her former costars on the hit Nickelodeon show ... Elizabeth Gillies and Matt Bennett . Check out the clip ... the Hollywood Arts High alumni are rocking out to one of Ariana's tracks ...