Who were these "18 or 19 others" who left the building right after the shooting?
I've never heard about a single person who was in the building who left shortly after the shooting. And certainly not "18 or 19".
The only person that I am aware of who was in the building at the time of the shooting and left afterwards - without permission from the police hours later - was Oswald.
On the contrary there, the wrongly accused left with full permission ---->
"Well, there was a billy love lady standing out there, he was on the
steps, see... And, Oswald was
coming out the door and he (Lovelady,
S.M.) said the police had stopped Oswald and sent him back in the
building, billy love lady said that Mr. Trudy (sic) told the policeman
that Oswald was alright, that he worked there, so Oswald
walked on
down the stairs."
*Source: Mr. Murphy (Sean) in 2010 sharing Mr. Jarman (James') House Select Committee on Assassinations testimony.