If you want to identify strangers based on a datbase of photos, what you are looking for is facial recognition or facial surveillance. It is not face detection.
Face detection is simply identifying and automatically capturing a picture of a face. It does not match it or alert on it. For example, a few IP camera suppliers include built in facial detection, but not necessarily face recognition.
As for facial recogniton / surveillance, make sure the supplier has experience in facial surveillance specifically. The difference is whether one is matching against controlled subjects (i.e., a person stares at the camera), vs a person just walking by (e.g., your request for strangers walking into the building).
I could not recommend any supplier that does this well.
But let me ask: how accurate does this need to be? how tolerant are you to mistakes? How much are you willing to spend per camera / door?