In the multiplayer there is a chat in the 3D depot and in OMSI to communicate with other players. Additionally there are shown some messages from the system.
Press T or click on "New message" to write a new message. With the Escape button you can stop the action and set the focus on the game again.
If you would like to have more information about a person in the chat, just click on the name of the player. You will then see an additional window with more details.
By clicking on the gear at the top, you can see the players who are online at the moment (alternative: hold the Tab key) and change the position of the chat.