To launch byobu automatically at login, on the command line, run byobu-enable. byobu-enable is a "wrapper script for enabling/disabling automatic startup of byobu after login into text console".
Otherwise run byobu and type F9 to bring up the "Byobu Configuration Menu". Arrow down to "Byobu currently does not launch at login (toggle on)" and hit enter. Byobu will now launch at login. Hit tab to focus Exit and hit enter to leave the configuration menu.
To launch byobu automatically at login within guake, open guake preferences. Check "Run command as a login shell". Now whenever you activate guake, byobu will be instantiated.