Again, not right. New people are not necessary at all. People are not in little bubbles with no communications between them. If there's even one person in common between two groups, then infection can/will in time cross between.

An infection travels like a game of 'telephone' passed from one to the next, right across the world.

Quarantine will initially regroup people, but in much smaller groups. If the disease is not yet widespread, then your new cohort will also likely be disease-free. There may be a tiny spike as an infected person in a large group joins a different, small group. But this is a good thing, since that group is smaller, and fewer people will now be at risk.

No, quarantine is the only effective way forward.