MMS with multiple recipients not creating group

[tomredman]

Hey all,

We have a paid plan, and an approved A2P 10DLC number through MessageBird. When I tried to send a group MMS – one that creates a group with 3 participants – it just sends two individual SMS (or two MMS' if I include an attachment, like an image):

var mb = require("messagebird");
const messagebird = mb.initClient(<redacted>);

var params = {
  recipients: ["15556667777", "15558889999"],
  originator: "15552223333", //our 10DLC number
  subject: "Check out this cool MMS",
  body: "Have you seen this logo?",
  mediaUrls: [
    "https://example.com/image.png",
  ],
};

messagebird.mms.create(params, function (err, response) {
  if (err) {
    return console.log(err);
  }
  console.log(response);
});

How can we use the API to create a group conversation with 3 participants via MMS?