To reply to a message, just click in the box below the message and type away! Or, you can click the arrow icon in the top right corner of the message you received.
If there are multiple recipients of the message and you want to respond to all of them, click Reply to all in the box below the message and begin typing.
You can also click the down arrow next to the Reply button and select Reply to all.
If you'd like "Reply to all" to be your default option, click the gear icon and choose Settings. Locate the Default reply behavior setting to change the option. Click Save Changes at the bottom of the page. You can still choose to reply to just one person in each message, but Reply to all will be the first option.
When you reply, the previous text from the rest of the conversation can be seen by clicking the Show trimmed content icon .