Currently, the only way to add an existing account to your Project without their knowledge or interaction is to use Quick Invite. Once you identify the account(s) you want and determine the Quick Invite’s settings, you’ll find two checkboxes under Advanced Options: Add to Pool only prevents the simplified email from being immediately sent; Convert Existing Users to Project Automatically identifies the existing accounts to be invited and immediately adds them to your Project without their interaction.
While the Quick Invite feature has the capability to add an existing account directly to your Project, it’s recommended that it be used very sparingly or under particular circumstances. Your customers or testers should be fully aware of their account and where their information is used. An email should be sent which invites the user to join, giving them the option. This process is extremely simple, enabling them to opt-in by simply clicking a link (which simply joins them to the Project once they've authenticated their Username and Password). Note that once invited, users can join the Project without using the email - the Project will be available on their Project Homepage under "Available Opportunities".
Alternatively, a new Source Type (Community Team Access) now provides open-access to selected Community Teams without sending an email. Similar to the prior method, this will show the Opportunity’s Title on the user's "Available Opportunities" list on their Community Homepage