as sungod stated. I have achieved this but is a very long winded process depending on how many sites and users you have.
parse all networks then iterate through each network to pull back the clients on a network
https://api.meraki.com/api/v1/networks/{networkId}/clients
You can add timescales onto this with appending something like the following on the url to set a time scale below brings back a 1000 user's per site for the last 5 hours.
?timespan=18000&perPage=1000
add the results into a variable to either filter on what your looking for or output to a csv to then query from the spreadsheet. I've found searching against Mac address easiest but getting harder to use mac with randomised mac's