You cannot do this achievement in the Capture The Flag game mode. You have to do this achievement in another game mode that consist of having you with a team of players. The recommended game mode is War and it must be a Ranked Match.
If you wish to find out more information in setting up a lobby, boosting towards the related achievement and using a Team Switch Glitch, check out this solution...Solution for General In Call of Duty 3
For this achievement we will assuming that you will be playing the War Game Mode. But only one of you among the group of 8 will be able to get this. Luckily though the online for this game is virtually dead so it is possible that you can get this achievement done at a quick pace.
When the match begins simply decide between all of you who is going for the highest score among everyone. You cannot have at least 2 people with the same score, otherwise the achievement will not unlock.
Once the winning player has been decided the team they are placed on need to win the match. Simply capture all of the objectives and get the winning player to either capture a flag by themselves or get some kills to increase there score among everyone else.
If done correctly the achievement will unlock at the Results Screen at the end of the match. Rinse and repeat for everyone else that needs it.