Olivia is 15. Her mother, Charlotte, told British media that her daughter is not answering her phone.
A friend called Sophie tweeted: "To my lovely best friend I hope you rest in peace my darling".
Taoiseach Enda Kenny said: "The city of Manchester has exceptionally close ties with our country, and I extend the solidarity of the Irish Government and all our people to those affected across the United Kingdom".
It's safe to say that hearing Grande's "One Last Time" will be a different experience for many of her fans from now on.
She said that police and security services believe they know the identity of the man responsible and are now working to establish whether he was acting alone or as part of a group.
U.S. President Donald Trump has been pressuring North Atlantic Treaty Organisation to do more to fight terrorism, and one of the most difficult decisions on his nine-day foreign trip could be how hard to push that point.
He added that the wellbeing of the songstress was also a priority, writing: "Also I am praying on peace of mind for Ariana".
As an investigation is under way, any photos or videos of the attack taken by witnesses could be extremely helpful to police.
Mrs May was speaking outside Downing Street after chairing an emergency Cobra meeting in the wake of the attack.
Pop concerts and nightclubs have been a terrorism target before. Most of the 89 dead in the November 2015 attacks in Paris were at the Bataclan concert hall, which gunman struck during a performance by Eagles of Death Metal.
Officials confirmed to The UK Sun the carnage is being investigated as a terrorist attack.
With his voice catching, James told the audience that he will continue praying for the victims and all of those affected by the bombing, which occurred when a lone male attacker set off an improvised explosive device. "So many young, beautiful, innocent people living and enjoying their lives murdered".