TL;DR

At the moment, only from Desktop: Select the Compose Icon > Select Create New Beeper Chat or Create New Beeper Group Chat. Search the contact by username.

How to start a chat with another Beeper user?

Step-by-Step

From your Beeper Desktop Client

1. Select the compose new message button
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2. Select Create New Beeper Chat or Create New Beeper Group Chat, depending on what you need.
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3. From there, you will be presented with the Direct Messages prompt
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4. Type the username of the other party, this format: @username:beeper.com
   • If you're creating a group chat, you will be greeted with the Create a private room prompt 
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   • Type a name, a topic (if you want one) and select whether you would be making a private or public room. A private room means that only people invited will be able to find this room. This setting can be change later.
5. Once you've filled all required information, you can press Create Room.