From B2evolution

The LDAP plugin provides authentication through one or more LDAP servers.

[edit] Setup

If you have problems with setting up the plugin, enable Debugging and see what gets outputted in the Debuglog category for the plugin (at the bottom of the page).

[edit] Location

The LDAP plugin got shipped with b2evolution until 1.9.1 and then has been moved out to the PluginsRepository. There has been no packaged release yet.

You can browse the source at http://evocms-plugins.svn.sourceforge.net/viewvc/evocms-plugins/ldap_plugin/ or check it out from SVN at https://evocms-plugins.svn.sourceforge.net/svnroot/evocms-plugins/ldap_plugin/.