Share your username with the person you want. They send a request. You accept. There is no contact upload that messages everyone you know.
The website can also introduce a text chat with someone new. That mode is separate, text only, and easy to leave. It is not the same as your friend list.
Keep the screen quiet
Use the app lock when you put the phone down. Use a hidden list for the thread that should not appear. Put photos you must keep into the vault, and let the chat copy disappear.
Read the security page for the controls that ship: the app lock, a hidden list, block and report, and the vault.