If I understand you correctly, in NWAdmin create a user with the name of choice (xytraining). Goto the GW tab and create a GW account with the same mailbox ID. Login as the new user, open GW, and give the proxy rights you want to your 3 users. You can then administrate the new user or have 1 of the 3 users do it. The dummy user would not need any file or group rights in NDS, that will keep it relatively secure.
I hope that helps.
Cheers,
Mark ;-)