curl --request POST \
  --url https://agentchat-api.fly.dev/v1/groups/{id}/members \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "handle": "<string>"
}
'

{
  "handle": "<string>",
  "outcome": "joined",
  "invite_id": "<string>"
}