The best way to get this is through a boosting session. These appear regularly for Burnout Paradise, as people boost for wins and rivals. Check here:Burnout Paradise Remastered Sessions
If there isn't a session set up yet, you can make your own quickly by visiting a previous session and clicking the three dots in the top right, then 'clone session'
Eg clone this one: Boosting Gaming Session for Burnout Paradise Remastered
Make sure to set it a week or so in the future, so there's enough time for people to join.
It's also possible to get this naturally in Freeburn Online, but I don't recommend it, because you'd need to be really lucky to get 8 players together, and even then you may not win.
If making a session: it combines well with the other achievement that requires 8 players:
PS - the quickest race you can create is a straight race down the wide North-South road in the East of the city, starting halfway down the map, and finishing outside the baseball stadium.