I'm seeing no option doing this on the Dashboard itself.
Guess I'd use the API to poll clients within the given network
(curl -L -H 'X-Cisco-Meraki-API-Key: <key>' -H 'Content-Type: application/json' -X GET 'https://api.meraki.com/api/v0/networks/{networkId}/clients')
compare this to the list of former known clients and sent out the MAC address of newly associated clients.
It should work that way, but it's not actually real-time.