Group messaging comes to Xbox One Preview and Windows 10 Xbox Beta App
Microsoft has rolled out an update to Xbox One Preview build which will enable group messaging enabling multiple gamers to participate in conversation. To make this even more convenient, the feature as expected is also rolled out to Xbox Beta App on Windows 10 App.

Those who can’t see the group messaging update, you will need to perform a hard reset aka complete power down reset by holding the power down button for five seconds until the console fully powers down, and then turns on. The issue is being looked on, and will get automatically fixed by 24 hours.
Here are the details as posted on the Xbox Forum
Group Messaging
Message your friends like never before with Group Messaging, now available on the Xbox One and the PC Xbox app! Start a new thread with multiple people in the same conversation or share activity feed content to existing conversations.

- Message friends about anything and everything!
- Share activities, achievements, game clips, and screenshots with others.
- Create new group conversations (just select more than one person when creating a new message).
- Create group conversations from 1 on 1 conversations (just select “Add more people”).
- Change the topic of your conversation in the conversation list.
- Add people to a conversation.
- Leave a conversation.
- Delete a conversation.
- Mute or unmute a conversation to stop or start receiving notifications.
- Select “View people” to view everyone participating in a conversation.
- If possible, redeem codes or launch websites from Xbox Live service messages.
Group Messaging Known Issues:
- People who don’t have access to Group Messaging (non-Preview users) will see your messages later.
- You may see a jump in your unread message count. To resolve this, navigate to those conversations to mark them as read (they’ll be near the end of your conversations list).
- When receiving voice messages and shared feed content messages from another user, you may not receive a notification.
