I'm playing the game and I've reached chapter 16 and the achievement says that I can recruit an extra employee, but I can only recruit one no matter how I try. I did choose artist in the beginning of my player background.
your character must have arts background. Khembish, Themba and Wolf each have an artifact tied to them. Normally, you can find the artifact of the one you first invited in the valley, and that will increase their stats.
In the arts background, you can select one of the other 2's artifacts to decorate the lobby, the artifact you choose will bring in that new guest.