There is a phpBB3 auth plugin that allows phpBB to authenticate on Crowd. It is released under LGPL and available at: http://sourceforge.net/projects/phpbb-atlassian/
Check the Download/Installation guide: http://forums.consultorpc.com/viewtopic.php?f=10&t=18
Support for this plugin can be found here: http://forums.consultorpc.com/viewforum.php?f=10