How do I add a new user to a domain administration group?      

The smsconfig command is used to alter/add users on the system controller.

smsconfig -a -u username -G admn <domain id>

So running the command ( as root ): 
#smsconfig -a -u mkc -G admn A
mkc has been added to the dmnaadmn group.
All privileges to domain A have been applied.

Would give user mkc administration access to domain A. 
Access lists can be viewed with the command:

#smsconfig -l <domain id>

The same user access can be removed with the command:
#smsconfig -r -u mkc -G admn A
mkc has been removed from the dmnaadmn group.
All privileges to domain A are now denied.