All user list
SELECT
SI_ID, SI_NAME, SI_USERFULLNAME, SI_DESCRIPTION, SI_LASTLOGONTIME, SI_KIND, SI_EMAIL_ADDRESS, SI_CREATION_TIME, SI_UPDATE_TS, SI_NAMEDUSER, SI_PASSWORDEXPIRE, SI_FORCE_PASSWORD_CHANGE from CI_APPOBJECTS, CI_SYSTEMOBJECTS, CI_INFOOBJECTS where (SI_KIND = 'User')
user with AD and Enterprise both
SELECT * FROM CI_SYSTEMOBJECTS WHERE SI_KIND='USER' AND SI_ALIASES.SI_TOTAL>1
To find the number of users in a group
SELECT SI_NAME,SI_GROUP_MEMBERS FROM CI_SYSTEMOBJECTS
WHERE SI_KIND = 'USERGROUP' AND SI_NAME='ADMINISTRATORS'
To extract all the users from specific user group
SELECT SI_ID, SI_NAME, SI_KIND, SI_USERGROUPS FROM CI_SYSTEMOBJECTS
WHERE DESCENDANTS("SI_NAME='USERGROUP-USER'", "SI_NAME='ADMINISTRATORS'")
check this thread as well