Though you have been assisted by our Chat Support, I wanted to make sure we shared our response here in the Community as well. At this time being able to create a private contact list is not an available feature. As a workaround we do have the option for creating a Campaign Creator whose role is restricted to creating new emails and editing existing emails. These user roles do not have access to your contact lists. We have also submitted this feature request to our engineers on your behalf.
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