OS user accounts are stored in /etc/passwd. You can get a list of all OS accounts using the following command:
cut -d: -f1 /etc/passwd
Furthermore, you can get a list of all OS accounts that had a UID > 1000 (not a service account) using the following command:
cat /etc/passwd | awk -F: '$3 > 1000 {print ;}' | cut -d: -f1
To list all SSO (Sguil/Squert/Kibana) accounts, you can use the so-user-list command:
sudo so-user-list
If you've enabled Elastic authentication, you can manage Elastic user accounts in Kibana under Management --> Users.