re: How does one go about joining a Mardi Gras Krewe?

Option 1:
Identify the crew you want to join and understand the costs. Get involved in the their fundraisers and build some relationships. Playing golf is helpful. If you're cool and they like you, you'll be asked if you'd like to apply for membership.

You'll get one shot so be prepared to pay.

Option 2: Take someone's place who can't make the parade and just ride (this is as much fun without the commitments).