Official comment
When you create a folder in Syncplicity it is stored where your default storage vault is located. If that's your on premise storage then that is where the folder is created. As an admin you can set folders to specific storage groups by manipulating the default group for your own account, then creating the folder when you have the default desired.
When you share a folder with others, all their contributions to the folder are placed in the same storage vault where the folder was created. Any new file or subfolder goes to the vault where the folder exists, regardless of what each user's current storage location settings are. It's the folder owner that determines which storage vault contains the folder. So you can control shared folders to be in on premise storage by controlling the default storage vault for the user.
Syncplicity support