The solution is to 1) create an intermediate file of ACTIVE users, then 2) do a lookup between the list of ALL users and ACTIVE USERS to get the INACTIVE users.
0) Have a list of all users uploaded to Splunk called "all-users"
1) Run this search to create the intermediate file named weekly_active_users.csv:
* | stats dc(userid) as "loggedin" by userid | outputlookup weekly_active_users.csv
2) Run this search to see the INACTIVE users:
| inputlookup "all-users" | lookup weekly_active_users.csv userid as userid | where isnull(loggedin)
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