How to store a collection of custom objects to an user.config file?

I would like to store a collection of custom objects in a user.config file and would like to add and remove items from the collection programmatically and then save the modified list back to the configuration file.

My items are of the following simple form:

class UserInfo
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }        
}

In my app.config I already created a custom section:

<configuration>
  <configSections>
    <section name="userInfo" type="UserInfoConfigurationHandler, MyProgram"/>

  </configSections>
  <userInfo>
    <User firstName="John" lastName="Doe" email="john@example.com" />
    <User firstName="Jane" lastName="Doe" email="jane@example.com" />
  </userInfo>

</configuration>

I am also able to read in the settings by implementing IConfigurationSectionHandler:

class UserInfoConfigurationHandler : IConfigurationSectionHandler
{
    public UserInfoConfigurationHandler() { }

    public object Create(object parent, object configContext, System.Xml.XmlNode section)
    {
        List<UserInfo> items = new List<UserInfo>();
        System.Xml.XmlNodeList processesNodes = section.SelectNodes("User");

        foreach (XmlNode processNode in processesNodes)
        {
            UserInfo item = new UserInfo();
            item.FirstName = processNode.Attributes["firstName"].InnerText;
            item.LastName = processNode.Attributes["lastName"].InnerText;
            item.Email = processNode.Attributes["email"].InnerText;
            items.Add(item);
        }
        return items;
    }
}

I did all this following this article. However, using this approach I'm only able