Every tab of Audience exports from its header. Contacts has no Add or Import: it is only people who signed up on your site, so a list of addresses goes to Leads (or a custom audience) instead.
Importing Leads
The Leads tab has Add and Import buttons. Leads you add stay out of All Users and are reached through the Form leads audience card, alongside the people who came in through a form. They have no behavioural history until the snippet sees them on your site, and they move to Contacts only when they sign up. Someone already in a custom audience is not added: they stay in their audience. Only leads you added or imported can be removed — a form lead comes back the next time the snippet sees them.
Your contact count is a plan limit checked against the live total, and imported leads count towards it too. A large import can push a workspace over its limit — check Settings → Usage against the number of rows in your file first.