Your staff will surely want to connect to the WiFi whilst at work but you don’t want to count them as visitors (they will end up looking like your most loyal customers!) so you can whitelist their devices.
The Mac address is listed as Wi-Fi Address.
On the Home screen, tap the Menu key and go to Settings
Scroll down and tap About Phone (or Tablet, if it’s a Tablet)
Tap Status and then scroll down to view WiFi Mac address
From the StartScreen, swipe left to display the App List.
Go to Settings > About > More Info.
The MAC address is listed in this section