As of yet you can:

• Host a P2P session visible to your friends with the Multiplayer Beacon
  
• Sync up to your hosting friend with the Multiplayer Sync Chip
  
• Travel together through time, each one using his own time machine.
  
• Visit each others safe houses.
  
• Visiting space together via dimension gate
  
• Exchange items
  
• Ranged combat
  
• Play Emotes on the Numpad
  
• You are able to play the story together, if you are at the same progress and quest at the same time, however at present only spawned enemies will sync up

Definitely planned:

• public p2p sessions
  
• traveling with another players car as passenger
  
• controlling the other players car (doing an owner switch)
  
• full syncing of quest relevant events
  
• synced Citizen
  
• synced Car Items (inserted fuel etc.)
  
• better overall performance for sessions with many players

HOST A LOBBY
(Credit goes to Neko for the screenshots)

Step 1:
-Craft a Multiplayer Beacon with the Mr. Crafty or spawn one in via Sandbox Mode.
-You can buy a Mr. Crafty in Story mode or get it from Edward later in the story.
-All necessary items can be gotten in the 6/12 store in Pine Lake, down the main road past the bridge from where you start from initially once out of the Prologue (or Sandbox).

Step 2:

Interact with it, once it blinks blue people can join your lobby!


JOIN A LOBBY

Step 1: Load a save of yours.



Step 2: Be friended with the person you want to play together on Steam.

Step 3: If the person has a session hosted and is in game, press on the join button and Voila you are both connected!


HOW TO SYNC UP

Step 1:
Craft a sync chip with the Mr. Crafty or spawn it in via Sandbox Mode.
Step 2.


Insert the chip into your car and interact with it to set it to a target player.
(Targets in the Ape Head Cluster or different timelines cannot be set, you have to go there manually first)

Time travel to your friend, enjoy!

Press enter to open the chat box and hold F1 to view the current player list/b]





See each other in time, explore the world together or hang out on the beach with a nice campfire, exchange items just like you would do in single player

Fights together against spawned npcs or mobs!

Display Emotes to talk to players, either by pressing the keys on the Numpad or typing them in chat
/Hello
/Point
/Threaten
/Sob
/Unconscious
/Dance
/Joy
/Yes
/No
/Laugh
/SitGround


Let your friends visit your safe houses!
Explore the Ape Head Cluster together!

Do car races,