(1) From within a project select the Messages Tile

Scroll through the messages (which are listed in reverse chronological order) to select the message you wish to reply to.

Clicking on a Message thread will display the original message, and any replies that have been received to that message.

Scrolling to the bottom of the message/reply thread will bring up a Write A Reply box (2)

When composing a reply you can add or remove recipients that will be notified (3)

And add attachments (4)

Selecting the bell icon from the bottom of the screen will take you into notifications. (5)

From here you can click into any message to read and/or reply to it.