Coldplay, Katy Perry and Justin Bieber are among the pop acts expected to join Grande on stage at the Old Trafford Cricket Ground on Sunday (4 June).
Proceeds raised will be used to help the families affected by the Manchester terror attack on 22 May, which left 22 people dead.
Outside Organisation CEO Dominic Mohan, a former editor of The Sun, told PRWeek the agency was appointed to a global PR brief "out of the blue" by US talent agency CAA, which represents Grande.
Mohan said: "Like everyone else, we had had a traumatic and disturbing week trying to process the horrific events in Manchester and they were still at the forefront of my mind as we got news of the proposed line-up - but what an act of defiance."
The concert, which is being coordinated by the likes of events company Live Nation (see tweet below) and ticket sales business Ticketmaster, was conceived by Grande and "masterminded" by her manager Scooter Braun, Mohan added.
However, Mohan conceded the concert had raised "difficult questions" about security, ticketing and whether the event was being held too soon after the attack.
In response, Mohan said: "These crisis points were high on our agenda. But once Manchester United footballer Michael Carrick generously agreed to bring his testimonial forward, the Greater Manchester Police gave the event its blessing. This was a vital moment."
He added that the concert had already received "an amazing reaction and support from the media and an outpouring of positivity from the people of Manchester".
Outside Organisation, which is handling PR on pro-bono basis for the concert, has recently added the likes of Iron Maiden, Alfie Boe and Boy Better Know to its roster.