Install InSync for each user ?
or
Using only one InSync (as a service ?) and giving him all the users accounts… and all the rights on all the folders ?
It should work fine on a Windows Terminal Server. Insync is a per user application. It installs itself on the user’s roaming profile and which is also where it stores its databases.
If you don’t want users being able to access each others Google Drive, ideally, per user would be the case