To be able to chat with other survivors, you'll have to complete a quest for getting a second character that will appear after level 100.  

 

 

After getting access to multiplayer, to add a friend you are going to need his nickname only.
Your nickname is unique and consists of two parts. The first one is the name chosen by yourself, while the second one is a 4-digit tag that will be given to you by the game. Your friend and you can choose one and the same name, but your tags will differ. The limit is 12 characters, you will be able to change the nickname once in 30 days.