You can get a permanent link to a message within a topic to share outside of Topicbox or to cross-reference a message within Topicbox, using the permalink icon.
When viewing a topic in the web interface, each message has a link icon in the header area. Use your browser's functionality to copy the link (usually this is via a right click to bring up a menu, then "Copy link" to place the link into your clipboard).
You can then copy the link into other applications, such as a mail client, or another browser window.
The link obeys the privacy settings of the original topic: copying a link to a message will require that anyone accessing the link pass all the group's permissions to view the content.