There are a number of airports where such procedures are in place.
In Amsterdam, all passengers on US-bound flights operated by US airlines are interviewed by security personnel before their boarding passes are scanned and they are allowed to proceed in a holding area immediately before the gate. As far as I can tell, these people are not affiliated with the airlines and are not wearing anything to identify their employer.
In Zurich I have been interviewed in the same way by someone wearing a United uniform before boarding a flight to the US. I was one of the last passenger to board so I went straight to the jetbridge and do not remember if there was a designed waiting area for passengers already interviewed (if there was one it was empty). The interview was much lighter than the Amsterdam ones.
In Madrid, an airline agent checking boarding passes "randomly selected" my travel companion for a secondary screening by another agent wearing the same uniform as airport security personnel. They checked his bag (opened it and swabbed it), and he underwent some additional questioning.
There is no such additional interview-type screening when travelling from Canada to US, or from Mexico to US.
On the whole, my sense is that these security agents are paid (directly or indirectly) by the airport, although they might work for third parties. The airline certainly has a presence there, but if they were airline employees they would be identified as such.
(I understand that El Al might be an exception and directly hire their own employees for additional security.)