One by one, Tony visited those left behind in the wake of Civil War and brought them or invited them to Stark Tower.
Except for Pepper, she was already there. Sort of.
And Betty. She just showed up one day.
And Shuri. She tagged along with T'Challa.
This is how the Protectors came to be in one place.
On a cold afternoon in New York the day after the Avengers were pardoned, Steve made the mistake of showing up at the tower - and Betty took a page out Sharon's book and got between Steve and Tony with a gun.
The Avengers thought that, by getting a pardon, everything would go back to normal. And in a way, they were right. Problem is, the new normal doesn't include them.