The Earth Dragon Balls were only capable of reviving one person at a time and that too, only once. Then how did they revive Vegeta and all the Namekians on planet Namek when the dragon was asked to revive everyone killed by Frieza?
Um... they weren't limited to only 1 person at a time.
It was the Namekian Dragon Balls that were limited to 1 at a time. The Earth Dragon Balls had no such restriction. In fact, they were used before to revive multiple people that Piccolo killed at the end of the Piccolo saga
Further reading : Dragon Ball