The best way to manage multiple clients/accounts and keep each data private is to use organizations.
Note: You must have a Venti or Agency account to add additional Organizations
To create a new organization within your existing account, just click on the Plan link in the upper right-hand drop-down menu.
From there, select the "organizations" tab to create a new Organization.
Now you can add specific members to each of your organizations and these members will only see the dashboards that were created within that org.