service/admin_users is a SAP Parameter attribute which is used to control Additional OS users authorized for system administration information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
Specifies additional operating system users that are authorized to
administer the system using the Web service interface of the start
service.
Specify the user names separated by spaces.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.