You can send a message to a group using your email client.
Use the email address of the group. You can find the email address of a group you belong to by logging in to the web interface and navigating to the group. The email address is in the group information panel on the right hand side.
The subject of your message becomes a group topic. Replying to an existing group message will add to the conversation about that topic.
It may take some time for your message to be delivered to the group if your group's messages are moderated. A moderated group means that messages need to be reviewed by a group moderator before being accepted.
Use the Topicbox composer
You can create a new message within a group by using the New Topic button. This will start a new message, prompting you for a subject and message content.
When you send your message, it still has to pass through any moderation controls before it will show up in the Topicbox web interface.
You can also reply to messages through the Topicbox web interface. Find the topic conversation you want to add to, and use the reply box to start a reply.
If you wish to refer to another message in your reply, select a piece of text. This will pop up a prompt "Insert Quotation in Reply". Clicking on that prompt box will copy the text into your reply. To cancel the quotation, unselect the text.
Drafts are saved: if you click away to another group or another set of topics, then return to your original topic chain, your unsent reply will still be waiting for you.