1. From within the DM you want to add someone to, tap the "Invite" button in the top-right corner

2. Copy the link and send it to someone else that you want to add to the conversation, or, if you're already connected on Volley with the person you want to add, select them from the "All Contacts on Volley" section just below the invite link

 

Note: If the existing DM is set up as a 1:1 DM and someone else is added, the existing 1:1 DM is converted into a group DM.

An additional note: within a space, 1:1 DMs cannot be promoted to a group DM. In order to create a group DM within a space, hit the blue plus button and follow the directions.