MAC addresses you add as clients don't show up until the client has connected, and then you have to display the list of clients for the time period the client would have been connected in.
Within the scope of the question you have asked, @kYutobi has given an excellent answer. Basically create a layer 3 firewall rule blocking all traffic, and then create a group policy and attach it to each individual client that overrides the firewall rules allowing the traffic for that one client.
HOWEVER, this is not a modern way of doing things. You should really consider using something like WPA2-Enterprise mode or at a minimum WPA2-PSK (with this last option being very simple to implement).
You could also consider using the "Trusted Access" feature of Systems Manager (although this does require you to buy Systems Manager licences). This uses certificate based authentication - but frees you from having to manage the certificates.
https://documentation.meraki.com/zGeneral_Administration/Cross-Platform_Content/Trusted_Access_for_S...
"Trusted Access" is still a little "green" at the moment. Apple support is good. Android and Windows 10 support is weak to poor - but give it maybe another 3 months and that should be sorted out.