I know about five people that went to Ariana Grande's concert, as far as I know they're all ok.
Such a senseless act, I can't imagine what it would've been like trying to get out. I think it raises a serious question about the security of such venues - they're incredibly hard to protect and if terrorists can cause such havoc without even entering the actual arena ...
I imagine in the coming days more information will emerge about both the attacker and the victims. Poor Ariana
RIP to those who have died