You have to do this achievement in the War Game Mode and it must be done in 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
This can be easily done in which everyone on the same team can get this achievement at the same time. But only one team will be able to get it at a time. Luckily though the online for this game mode 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 which team is going to win the match. Once that has been decided, get everyone on that team to capture the objectives and leave the last one until everyone is nearby that flag. The opponents can have a break and watch the flags rise to their doom. When everyone has arrived at the final flag, you all need to capture it as a team. Doing this will register everyone as capturing the flag at the same time instead of a single person getting the credit.
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.