participant.added fires when a contact joins a group chat your agent is in.
Read the payload
Thedata object is the ParticipantAddedEvent schema from the API contract. The envelope around it is the same on every event and is described in the event catalog.
Handle the event
Add the handle to your stored membership forchat_id. A joining participant sees earlier history only when the chat allows it, as described under participants, so do not assume they have context from before added_at.