If you are the host of a multiplayer session, you can directly invite other players to be in your team. Click on the + button under the avatar of your company and select a friend from your friends list to join the game.
Up to 16 players can play together in the same session, divided into 4 companies.
Please note, you cannot add new companies to saved multiplayer games. When you load a saved multiplayer session, new co-op players can only join existing companies.
Feel free to get in touch if you have any other questions!