Depends on the platform where you have mantis running but we have integrated logon based upon AD (where mantis is on a windows server).
The plugin can be found here:
http://www.mantisbt.org/bugs/view.php?id=12627

To make this possible I change the following core/ldap_api.php lines (273 - 299) and define this in config_inc.php page $g_ldap_group_dn = 'groupMember';.
I can't push this to mantis forge because of this message
{{{
error: Cannot access URL http://git.mantisforge.org/r/mantisbt/gtz-et.git/, return code 22
fatal: git-http-push failed
}}}
function ldap_authenticate( $p_user_id, $p_password ) {

if password is empty and ldap allows anonymous login, then

# the user will be able to login, hence, we need to check
# for this special case.
$t_ldap_organization = config_get( 'ldap_organization' );
$t_ldap_root_dn = config_get( 'ldap_root_dn' );
$t_ldap_group_dn = config_get('ldap_group_dn');

$t_username = user_get_field( $p_user_id, 'username' );
$t_ldap_uid_field = config_get( 'ldap_uid_field', 'uid' );
$t_username = user_get_field( $p_user_id, 'username' );
$t_search_filter = "(&(uid=$t_username)($t_ldap_group_dn))";
$t_search_attrs = array(
    $t_ldap_uid_field,
    'dn',
);
$t_ds = ldap_connect_bind();

log_event( LOG_LDAP, "Searching for $t_search_filter" );
$t_sr = ldap_search( $t_ds, $t_ldap_root_dn, $t_search_filter, $t_search_attrs );
$t_entries = ldap_count_entries( $t_ds, $t_sr );
ldap_free_result( $t_sr );
ldap_unbind( $t_ds );

if ( is_blank( $p_password ) || $t_entries == 0 ) {
    return false;
}

return ldap_authenticate_by_username( $t_username, $p_password );

}

Please if you have any advise to use LDAP group authentication without modifying the core page comment me here.

Thanks