‘identifier’ is a table column that stores an identifier for the user. (Steam ID, IP, etc.). You need to check the ‘group’ column to see if it equals ‘user’. What you are currently doing is checking the ‘identifier’ column and seeing if it equals the ‘group’ of the source user.
You would need something like (not tested but hopefully you get the idea)
MySQL:executeQuery("UPDATE users SET `job`='1' WHERE `identifier` = '@identifier'
AND `group` = 'user'", {['@identifier'] = Users[source].identifier})