Complex sentence: For businesses that want additional security, the private-cloud plan allows the customer to manager the user accounts with their own server, thus all account and file data are controlled by the customer, and no information is stored in the internet cloud.
Split into simpler sentences: For businesses that want additional security, the private-cloud plan allows the customer to manager the user accounts with their own server. Since the server manages the user accounts, thus all accounts and data are controlled by the customer and no information is stored in the internet cloud.