Once screen sharing work has been completed, Guacamole should have the foundation necessary to support recording of a shared screen.
As originally stated in a forum post, the only mechanism for this previously was to record raw Guacamole protocol data:
If you want to make a video of a Guacamole session, the best way would be to record the Guacamole protocol between guacd and the web application (along with timing information), and translate that into video.
As mentioned in comments on this issue, mechanisms exist for recording VNC directly as well, but as these will involve additional VNC connections (and thus more bandwidth), handling the duplication of connections within guacd would be more efficient. This is, in fact, one of the main driving forces behind screen sharing at the Guacamole level.