Unlike Touch ID in which users could register multiple fingerprints and even fingerprints of other users, Face ID was not designed that way. Even Apple claimed that Face ID was not designed to support multiple users presumably due to security reasons. However it seems that the upcoming iOS 12 update will change that.
While it was not mentioned by Apple during their presentation, it seems that users who have downloaded the beta have discovered this new feature. AppleInsider has also since confirmed that it works, where within iOS 12 there is a settings that allows users to add another face in Face ID.
However it should be noted that this doesn’t mean that Apple caved into the demands of its users, but rather this was designed so that users could provide an alternative look so that Face ID can recognize them. As AppleInsider notes, users with strong prescription glasses might run into issue with Face ID as their lenses might distort the laser dot pattern emitted by the TrueDepth camera.
By allowing users to provide an alternative look, it should help address the problem, although it has since been tested and noted that it works with completely different faces and also when the genders are not the same.