FudForum has a PHP "hook" for integration with other login systems. We would need to look at this in more detail to understand how easy it is to use: http://cvs.prohost.org/index.php/Login_integration
FudForum also has a plugin system: http://cvs.prohost.org/index.php/Plugin
Ldap.plugin is a plugin that will authenticate users against an LDAP (Lightweight Directory Access Protocol) directory server and not against FUDforum's user database. This plugin ships with FUDforum 2.8.1 and later releases. http://cvs.prohost.org/index.php/Ldap.plugin
Note: FUDForum does have support for arbitrary groups.
A php client for crowd exists, http://confluence.atlassian.com/display/CROWDEXT/Integrate+Crowd+with+PHP