Baldur's Gate 3 can be played as a singleplayer RPG, where it's just you and your NPC companions, but you can also play it in a co-op group of 2-4 players. Multiplayer works even in the Early Access version, although there are some quirks you should know about before you set up a game.
Here's how to start a new co-op game so that all of the players get to create custom characters, as well as how to invite friends to an existing singleplayer game.
How to start a new multiplayer game (recommended)
To start a Baldur's Gate 3 co-op game from the beginning with custom characters, hit the Multiplayer button from the main menu. From there, select the Create button at the bottom of the screen to make a lobby. Change the "Who can join your game" setting to Invitation Only or Friends Only. Once you do that, you can click on the empty player boxes to invite players from your Steam friends list.
(Alternatively, you can send the Server ID to friends, and they can use it with the Direct Connect option from the Multiplayer screen.)
When you press Launch, everyone who's connected will be sent to the character creation screen, and then awakened together at the start of the game.
What happens if a player disconnects: When a player leaves the game, control of their character is assigned to someone who is still connected. The host can adjust character assignments by hitting Escape and opening the Session menu.
How to save and resume a co-op game: The host can save the game at any time. To resume the game later, the host must load one of those saves, just like they would if they were loading a singleplayer game. Once in the game, the host should press Escape and open the Session menu. From there, they can hit the Settings button at the top right and open the lobby to invites (by default it's open to nobody). After that, they can invite the other players and use the up and down arrows to assign them control of their characters once they connect.
How to invite players to a singleplayer game
Instead of starting a new game together, you can invite friends to join a singleplayer game in progress. Instead of creating their own custom characters, they'll take control of companion characters you've recruited to your party.
The method is the same as resuming a co-op game that was started with custom characters. Load a save, hit Escape, and open the Session menu. Open the Settings and open the game to invites. After that, you'll be able to invite Steam friends to the game.
If there isn't a companion available for a player to take control of, the invite won't work. And once another player takes control of one of your companions, you won't be able to talk to them as if they're still an NPC. At least in Early Access, they will behave just like a custom player character.
How Baldur's Gate 3 co-op works
Players can more-or-less do their own things. If one player starts a conversation with an NPC, the other players can watch it and suggest dialogue options by clicking on them. They can also wait for the NPC to free up and then have the conversation themselves, unless the story has moved on. Or they can wander off and get into fights on their own, ignoring their companions.
It's not super elegant right now, but co-op should improve with new features throughout Early Access. Larian talks about some of its plans in the video below:
Bugs and issues
Despite the recent hotfix, you're going to encounter bugs—Baldur's Gate 3 is in Early Access, after all, and it's not as if Larian RPGs are bug-free even when they're not in Early Access. I have two pieces of advice, which together are the most commonly given pieces of computer-related advice: Save often, and if something breaks, turn it off and back on again.
When Andy and I tested co-op, we ended up getting stuck in a fight where neither of us could end our character's turns. The only fix was to load an old save and try the fight again. It worked fine on the second attempt.
If you want to report a bug to Larian, you can do so in the official forums.