Short answer: Find lots of people online to play and be in a crew with! The game starts you off in a random person crew with others who have recently played, but there is no guarantee any of them will stick around.
Long explanation: The amount of rep (experience) a crew has collected to reach rank 50 is far more than an individual needs for their personal level. As of the time of this writing, me and my friends are almost crew rank 45. Not too much farther to go. Any rep you earn is also given to the crew in an equal amount. You get a bonus to money and / or rep for racing with others online. If you want, shoot me a message and I can just add you to our crew. By the time you ask, we may have it maxed already. If not, you'll just be helping us get it there faster. Being in a crew doesn't decrease your personal levels at all. If racing with other people, you still get the same rewards as you would for placing first if you beat all of the NPCs in the race, but are behind your buds. Being in a crew actually nets a flat bonus as well to your money, rep, and heat gains.
You can do this all solo if you wish, but you will be in for a large world of race grinding later on long after you finish the game, every race, every collectible, etc. And again, you can also just stick with the default crew they put you in, but there is no guarantee anyone else stuck in there will play regularly or anything.