Avoid no-member user discussion partecipation in private community

Hi, I want to use Viva Engage but when I create a private community I noticed that in a discussion a member can mention or add a user who is not a member of the community! This non-member user gets an email with which he views the specific discussion and can participate by making a contribution. But what if I want to prevent non-member users from being added in discussions? Is there any way to disable this feature which moreover conflicts with the definition of a private community in Viva Engage where "only approved members can view and participate in the discussion"?