Unfortunately we do not currently have a way to accomplish that. We realize there is a huge need for this and we are working on a way to provide this functionality, but we do not anticipate to have anything out until at least the 9.1 release.
You can leverage self-registration to apply a default desktop to new users self-registering through the client portal. You can also apply desktops to individuals through the Desktops tab on their user profiles in Admin. You can also submit a request to email@example.com for a custom, 1-time database job to apply desktops to a group of users.
Apologies for the inconvenience!