This article explains how to add a current Wdesk user to multiple accounts.
Working with General Security Settings
From the admin panel, choose the Security tab. Under the General sub-tab, click Edit Settings.
In the window that appears, enter the domains you want to add. Make sure the domains are all lowercase. If you are adding multiple domains, separate them with a comma. Click Save Settings to complete the addition.
Adding Email Domains
Once these domains are set, only users with emails using these domains will be allowed. Adding a user to multiple accounts requires that both accounts allow the desired user's domains.
Note: Admins with the same email domain as the member they are adding do not have to add the domain.
Once the new domains are saved, you can now add a user with the new email domains to your account. For detailed information on adding users to your account, see Managing Members and Groups.