Once a Conversation has been created, any Participant can add new Participants, or remove current ones. You can also remove yourself from a Conversation.
If viewing the Conversation in the web app, first click on any of the Participant circle icons at the top
You'll then see the Participants panel. Click the "Edit" button to get into edit mode
You're then to free add & remove Participants as needed, using the "Search or Add Email" field,, the previous Participants list, and the red 'x' icons
All newly added Participants will receive an invite email.
If you want to remove yourself from a Conversation, you can click the red 'x' icon next to your name in editing mode, or the "Leave Conversation" button in the Participants panel
Please note that there is no current way to delete a Conversation, but removing yourself effectively does that for you. Owners will NOT be able to leave a Conversation, however.