Starting a conversation with any of your peers is simple! You can first visit their profile, finding the "Contact" button as seen below:
Alternatively through the Conversations page (accessed by the envelope icon), you can start a new conversation with the (+) icon:
You can simply start by typing your peer's name and they should appear in the list with their profile ID #.
Do note that it is also possible to add multiple recipients to your messages!
For more information on how Conversations work.