For domain administrators (with access to the identity tab), it is now possible to copy users from one accounting unit (site) to another.
In the main menu, click on administration button:
Go to Offices and Users:
Select the office where you want to copy the user to, and click the copy button:
Select accounting unit (site), the office and the name of the user. Click save.
The user will be created in the new site. User will retain the same password and username as before, but will now have access to this site as well. Remember to add relevant rights to the user in settings.