I would take the assumption that the syntax of the BESUserAdminClient command is incorrect. To confirm I have posted the switches and example below. To confirm all the settings and installation requirements I will need to know what the exact version of BES and Domino you are running. Also if you could post back the exact command that you are running to add users it will help us resolve your issue.
1.At the command prompt, switch to the folder in which the BESUserAdminClient.exe file resides.
2.Type BESUserAdminClient -add -g with the following options:
-p <client_password>
-u <user_name>
-b <BlackBerry_Enterprise_Server_service_name>
-group <group_name>
-sqluser <SQL_username> (if you are using SQL authentication)
-sqlpass <SQL_password> (if you are using SQL authentication)
-n <network_address of the BESUserAdminService> (if the BESUserAdminService and BESUserAdminClient are not running on the same computer)
Domino Example:
BESUserAdminClient -add -p password1 -u “CN=Gary Cutri/O=Test” -b server01 -pin xxxxxxx -it_policy “Default”