If you have a secure network you can also add Domain Admins and Domain users.
With recent Microsoft security patches we have seen in some cases a need to add Network Users for Access and Launch Permissions.
Specifically we see a need to sometimes Enable "Network Access: Let everyone permissions apply to anonymous users" and set "Network Access: Sharing and Security Model for local accounts" to "classic - local users authenticate as themselves".
We have found that for reasons only known to Microsoft we sometimes need to set Local Securty Policies.
You should set this at the computer level and not the application level.
To open security wide, which should only be done on a secure network, You need to have System and Everyone with Launch and access permissions on all PC's involved in the OPC connection.
If you are using a WG(WorkGroup) or WG to Domain or Domain WG you need to make sure the OPC users in the Domain are built into the local security account of the WG PC(s).
It is intended for use in Domains which have a central Access Control List.
COM/DCOM, which is what OPC uses as a core technology, does not work well, and usually not at all, across multiple domains.
If that does not work then you can always go the rout of opening DCOM as wide as possible. Remember these files need to be properly registerd on both PC's.
Reboot and try to connect to the server again.
Click OK and type exit to close the command prompt window.
A dialog box should appear saying: DllRegisterServer in oleaut32.dll succeeded.
A dialog box should appear saying: DllRegisterServer in ole32.dll succeeded.