The pirates of the Crimson Fleet are the first enemies you face in Starfield, which makes it a bit surprising when you find out they're a faction you can join. Alongside goody two-shoes like the UC Vanguard and Constellation, you can sign up to be a baddie no-shoes who wears an outfit with a skull on it so that nobody forgets you're batting for Team Evil. This is a Bethesda RPG after all, and if you couldn't join the Dark Brotherhood and the Thieves Guild then Oblivion wouldn't be half as fun.
You don't have to be on your "morally questionable" playthrough to join the Crimson Fleet, though. The questline begins with you signing on as an undercover agent, and there are allowances for playing without killing a bunch of innocents, though you can flip out and go full pirate if you want. Here's how to start the Crimson Fleet questline.
How to join the Crimson Fleet
There are two straightforward ways to sign up with the galaxy's blood-red buccaneers. The easiest is to get arrested in UC-controlled space. Walk up to a security officer in New Atlantis on Jemison, take a swing at them, and you'll be off, though it's probably worth sending your active companion away first if you don't want to take the hit to your affinity score. The arresting officer will tell you they have special orders and you'll be transported to a UC ship called the Vanguard, where Commander Ikande will offer to wipe your slate clean if you agree to go undercover.
If you don't want to sully yourself by committing a crime, you can also begin the questline by joining the UC Vanguard legit. Talk to Commander Tuala in the MAST building in New Atlantis, and he'll explain that you can join the United Colonies' weekend warrior squad by passing a piloting simulation. You'll need to defeat three waves of digital enemies, which should be easy enough even if you haven't sunk a lot of points into piloting. The more waves you defeat the better the reward you'll get for signing on, however.
The first mission in the Vanguard storyline sends you to Tau Ceti to investigate a facility that's gone silent. After that you'll be given two objectives: one lets you continue the Vanguard plot, the other sends you to the UC Vanguard, where you'll be given a less-threatening version of Commander Ikande's recruitment speech and become an undercover operative working for SysDef.
Whichever way you choose to begin the Crimson Fleet questline, the next step will be to take a cargo of illegal drugs that SysDef has lying around to Cydonia on Mars, find a shady lady called Saoirse Bowden at the Trade Authority, and convince her you're a smuggler. She'll point you in the direction of Adler Kemp at the Broken Spear, who has a contact in the Crimson Fleet. To get Kemp to trust you, you'll need to shake down someone who owes him money, and if you don't want to get violent a persuasion check will solve the problem.
Kemp sends you to Europa to meet with Astraea, an actual pirate at last rather than just a dodgy geezer who happens to do business with them. Once again your loyalty will be questioned and you'll be given a quest to prove yourself. This time, you're expected to deal with an ex-pirate called Austin Rake. She'd like you to turn him into more of an ex-parrot, if you catch my drift.
Again, there's a non-violent solution if you don't want to blow up the medical supply ship Rake's hiding out on. However you resolve the situation, when you return to Astraea's location her ship's under attack from Ecliptic Mercenaries, providing you with an opportunity to come to her rescue and fully prove your worth. Do that and she'll trust you enough to furnish an invitation to the Crimson Fleet's HQ, where you'll finally be able to join them properly, and score a Pirate Swashbuckler Gear outfit that gives a +5% bonus to critical damage while you're at it. Yo ho.