When you add new contacts to your account, you'll need to be sure that they are assigned to a check in button. That assignment is what makes an employee contact available for visitors to see and notify when they arrive.
You can assign your contacts within the employee contact settings directly in the Button Assignment section:
If you have added more than a few new contacts, you can also assign them to a button via button settings, rather than one by one. Click on 'visitor buttons' on the right then 'edit' for the button you want to assign your contacts to. Scroll down to the employee contacts area, scroll down the contacts list to check the boxes to add your newly loaded employees to your button.