How do I disable notifications for a group chat?

In your group chatroom, tap the triple-lined menu on the top-right corner and you will see a bell icon on the bottom. Tap the bell icon to turn off notifications and you will no longer receive notifications from that chatroom. If you wish to turn on the notifications, just tap the bell icon. 

If you turn off notifications for a group chatroom, a crossed-out bell icon will appear in your chatrooms list.