Isso e muito simples e existem muitos artigos a falar disso.
Primeiro precisa fazer download do Client object model API, onde se encontram a DLL para comunicar com o SharePoint.
http://www.microsoft.com/en-us/download/details.aspx?id=21786
Depois precisa fazer a referencia ao dll "Microsoft.SharePoint.Client.dll" e "Microsoft.SharePoint.Client.Runtime.dll" e quando o asp.net fazer a chamada no botao autenticar no site e fazer o codigo restante...
codigo exemplo:
// Starting with ClientContext, the constructor requires a URL to the
// server running SharePoint.
ClientContext context = new ClientContext("http://SiteUrl");
// Assume that the web has a list named "Announcements".
List announcementsList = context.Web.Lists.GetByTitle("Announcements");
// We are just creating a regular list item, so we don't need to
// set any properties. If we wanted to create a new folder, for
// example, we would have to set properties such as
// UnderlyingObjectType to FileSystemObjectType.Folder.
ListItemCreationInformation itemCreateInfo = new ListItemCreationInformation();
ListItem newItem = announcementsList.AddItem(itemCreateInfo);
newItem["Title"] = "My New Item!";
newItem["Body"] = "Hello World!";
newItem.Update();
context.ExecuteQuery();
André Lage Microsoft SharePoint, CRM Consultant
Blog:http://aaclage.blogspot.com
Codeplex:http://spupload.codeplex.com/http://simplecamlsearch.codeplex.com/
