今天遇到了同样的问题，但与DirectoryServices没有太大的不同。下面是与Active Directory一起使用的修改函数

public bool AddUserToGroup(LdapConnection conn, System.String userdn, System.String groupdn)
        {

            // modifications for group
            LdapModification[] modGroup = new LdapModification[1];

            // Add modifications to modGroup
            LdapAttribute member = new LdapAttribute("member", userdn);
            modGroup[0] = new LdapModification(LdapModification.Add, member);
            try
            {
                // Modify the group's attributes
                conn.Modify(groupdn, modGroup);
                System.Console.Out.WriteLine("Modified the group's attribute.");
            }
            catch (LdapException e)
            {
                System.Console.Out.WriteLine("Failed to modify group's attributes: " + e.LdapErrorMessage);
                doCleanup(conn, userdn, groupdn);
                return false;
            }
            catch (Exception e)
            {
                Console.WriteLine("Error:" + e.Message);
                return false;
            }
            return true;
        }