You are right that the Registry key mentioned in your post is required for each user. To make this available to all users, add the key value to the hive HKEY_LOCAL_MACHINE rather than HKEY_CURRENT_USER. This will be possible if the GroupWise client is version 6.5 SP1 or later.
We will shortly be releasing a minor update to the Formativ installer, to write the key value to the appropriate Registry hive automatically.
I hope this helps.