Since the Game is now over 5 Years old and there are now not so much people playing this game, I think it would be a good time for a Boosting Guide:
This Guide it for all the five Challenge-Achievements:
The People Person achievement in Burnout Paradise worth 69 pointsAwarded when you complete all 10 Island Freeburn Challenges. These can be completed by 2 - 8 players
To boost them all in only one huge Trueachievements-Session, follow this steps:
1. Create a 8-Player Session.
2. When you have enough Player you have to choose 3 Sup-Leader!!!
3. When the Session starts do ALL the 8-Player Challenges! (Car-Freeburn, Car-Time, Car-Island & Bike-Time) 
4. Split all Player to two 4-Player Groups and do all the 4-Player Challenges (Car-Freeburn, Car-Time & Bike-Time) 
5. Move one Player from on Group to the other. The 3 Player Group do all the 3-Player Challenges (Car-Freeburn, Car-Time & Bike-Time) and the 5 Player Group do all the 5-Player Challenges (Car-Freeburn, Car-Time & Bike-Time) 
6. Split all Player to four 2-Player Groups and do all the 2 Player Challenges (Car-Freeburn, Car-Time & Bike-Time) 
Note 1: This will take a bunch of time! May you need to create a second Session with the same Players.
Note 2: This will also work if not all Player have the Big Surf-Island DLC. But all player they don’t own the DLC have to wait for the other Player to complete the Island Challenges!
I know my English is not that good but I hope everyone understand what I mean!