Hello @IWilsonR,
I found a question on SplunkAnswers which is very similar to yours: Account Creation And Deletion within a given time.
Try this search using the transaction command:
index=Linux_os eventtype="linux_sec"
| transaction user startswith=eventtype=useradd endswith=eventtype=userdel maxevents=2
| where duration<24*3600
This should work too:
index=Linux_os eventtype="linux_sec"
| transaction user startswith=eventtype=useradd endswith=eventtype=userdel maxevents=2 maxspan=24h
... View more