The Crew

My progress towards this achievement got reset at 89,234 points, which prompted me to investigate how this achievement actually works, and to write the below guide, which I hope is of use to anyone with the time to read it!

Introduction
This achievement requires you to earn 100,000 Reputation points (RP) but it's very important to note that this is not a cumulative amount over the lifetime of your career on The Crew. Instead, you must earn all 100,000 in a single calendar month.

This is because the Reputation Points and Faction Ranks of all players are reset by Ubisoft on the 1st of each month as part of their 'Faction War' community event - so that everyone starts on an even playing field for each month's war. So, if you have 72,000 by 31st of January for example, you'll find yourself back to 0 on February 1st.

A further thing to note is that all missions and PvP races do not award any reputation points on the last 2 days of the month, while the winning 'faction' in the faction war enjoy playing some exclusive bonus missions.

This means that, for example, in February you only have up until the 26th to earn your RP, in March you'll have up to the 29th, etc.

So, on average (and excluding February), you probably have 28 calendar days a month to get 100,000 points. That's an average of 3,571 RP per day.

It is therefore advisable to start working on this achievement as close to the start of a month as possible in order to give yourself time to finish it.

Earning Reputation Points
To earn reputation points you can do 3 things:

• Play PvP Races
• Complete faction missions
• Send Your Friends on Faction Missions

Your time will be best spent combining playing PvP Races and sending Friends on faction Missions.

Doing faction missions yourself takes way too much time for a very poor RP return and, given the instability of the server connections in The Crew, you could find yourself losing connection halfway through an hour long race and have 0 RP to show for your time.

PvP Races
Firstly, ensure that you max out the 'Notorious' perk from Roxanne. When maxed out (5 perk points) this perk means that you gain 10% more reputation points from PVP events.

Most PvP races will take between 80 seconds and 8 minutes and reward between 280 and 387 RP per race based upon your finishing position and the number of perk points you've put into the Notorious perk.

Sprints are the shortest races and always have the word Sprint in brackets at the end of the event name, such as: Keys to Little Havana (Sprint) - which can be found in the South PvP lobby.

If you are the leader in your lobby (i.e. the person with the most points) you can keep choosing the short events. Alternatively, you can get 2 to 3 of you together in a crew, join a free-for-all lobby and between you all keep getting a high placed finish and choosing the 'Sprint' events.

Sending Friends on Faction Missions
When it comes to sending friends on faction Missions, this is a way for you to earn quick, free, easy reputation points. Simply follow these steps:

• While playing the game, add any Level 50 players you see as a friend. You can do this by pressing right stick, scrolling to their Gamercard, selecting ‘View Gamer Profile’ then pressing Add Friend.
• Open the world map.
• Select a faction mission and choose ‘Send friends’
• You will be prompted to choose which friends you want to complete the mission.
• Each level has a ‘Chance of Success’ and it’s your goal to select enough friends ( up to 4) to make that chance of success 100%.
• You can then launch the mission.
• The mission will then become greyed out on your map and, in-real time you will eventually be notified that the mission has been completed – i.e. if it’s a 10 minute mission then you will get your notification in 10 minutes time.
• The mission icon with have a green exclamation mark on it. Select the mission then select ‘See Result’.
• You now collect your reward (Bucks and RP).
• Then just send your friends on the mission again.

Sending friends on missions rewards between 20 and 150 RP and you can have friends out on 9 missions at any one time - you'll obviously earn points quicker if you send friends on missions that reward 150 RP rather than those that reward just 20 RP.

Note that if you max out the 'Friends With Benefits' perk from Roxanne, you can send up to 4 friends on each mission to ensure you get the 100% chance of success.

Neither you or your friends have to do anything (they won’t even know you’re sending them on missions). Free reputation points!

Summary

If you make sending friends on faction missions a daily routine you can easily pick up at least 1100 RP a day. If you can then do an average of 9 PVP races a day as well then you'll get at least another 2,700 RP. Do that every day for 28 days and you'll have the achievement.

A good working pattern is to send your friends on faction missions and do a few hours of PVP. By the end of your PVP session you're friends would have finished the faction missions and you can send them off again.

However, the earlier you can get points the better, so if you have free weekends or days off from work/college/school - I would hammer away at as many PvP races as you can!

Notes from the comments section:
* Lambent x Bunny, on 19th March 2015, confirmed that the RP rewards for sending your friends on Faction Missions has been increased, which should help speed up progress toward this achievement.

This guide is the Easier way to get this achievement without playing any PVP events.
Important to remember that every month reputation points and award progress resets and the last two days of the month don't count.
First,go to perks and unlock all 3/3 "friends with benefits" from Roxanne so you can send 4 friends on a fraction mission.
Second, go to perks again and unlock all 5/5 "notorious" from Roxanne to obtain 10% reputation from fractions.
Third, Get 5 or more level 50s and at least 4 friends or more friends that are level 50 under 50.
Now send your friends on those fractions

Frontier tour Location North of Chicago
showtime Location Miami in the keys
Landmark tour Location Las Vagas
The longest course Location very south of Texas
Highway child Location Los Angeles
And the rest of then on random fractions

With those above you should earn 4000 reputation points. Note it takes 12 hours for one cycle and you are going to do this every 12. So at 12 am is one cycle and 12 pm is another cycle. You should earn 8000 in 24 hours. Within 13 days you will earn 104000 reputation points without a single PVP event.

Reputation points can only be earned in public PvP modes and faction events. Both of which are unlocked early in the game by progressing through the story and levelling up. Missions and races normally award reputation points in small amounts, typically 100-300 rep points depending on the event. A quick faction mission to complete is “Way Out West” located south of Las Vegas in the Mountain States. You must be level 40 and own a raid vehicle, preferably car level 700+. The mission can be completed within a minute and offers 200RP, or 220RP if you max out the perk that increases reputation points from faction and PvP missions.

-It should also be noted that reputation points reset as well as the award progress on a monthly basis (thanks SpectralSprintr)

EDIT 09/01/15: Apparently the patch on 07/01/15 has removed this faction mission from the game because..well it's Ubisoft. I'll update solution shortly to the next best method of earning rep points, please bear with me.

I want to add that any RP you earn playing PVP in The Summit does NOT count towards this achievement.

I didn't notice this until I devoted an entire day playing PVP solely at The Summit. I checked my progress at the beginning of the day. When I checked my progress at the end of the day, it hadn't budged one bit.

You can check your progress by going to the map, clicking on the left stick, and scrolling over to the Awards tab. Once at the Awards tab, press on the PvP icon. On the PvP page, scroll to the Faction A icon and press . The third row of icons on the Faction A page is devoted entirely to RP points earned so far.

Good Luck!

This more an update than a solution.

Not sure since when it changed but sending friends to do the missions now grant you a maximum of 900 RP for the long ones(More than 2 hours), 450 RP for the not so long ones(More than 1 hour) and 250 RP for the shorts ones (10-60 minutes).

There are 9 faction missions that last 12 hours and 1 that gets completed after 2 hours.

So you end up with a little bit over 6,000 RP every 12 hours or 12,000 RP every 24 hours as base and if you grind the short (250 RP - 2 hours faction mission) you can get over 15,000 RP every 24 hours.

If you are constant you can unlock this achievement in 9 days.

On the assumption you're done mostly everything else (good cars, max perks) this is theoretically possible in 20hrs.
North of Los Angeles is a race called Leaving LA you will get 825 Rep in Sub 12 mins (I got from about 11:15-11:45 per race) so say 10 in 2 hrs gets you 8250.
Send your friends on Orange 1+hr missions (225 REP) of which I think there are 7 max every two hours plus 3 on 12hr 900 REP missions. That should get you 10% every two hours.