At some point of time there might be a requirement to access a SharePoint site programmatically and list the names of all the groups a user is associated to without using SharePoint APIs. Here is a small piece of code in C# which demonstrates how to achieve the above action using web service(s) available for a SharePoint site. First you need to add a reference of the UserGroup web service to your solution.

try
 {
      UserGroup.UserGroup userGrp = new TestApplication.UserGroup.UserGroup();
      userGrp.Url = "http://SPSite/_vti_bin/UserGroup.asmx";
      userGrp.PreAuthenticate = false;
      userGrp.UseDefaultCredentials = true;
 
      XmlNode node = userGrp.GetGroupCollectionFromUser("userLoginName");
      XmlDocument xmlDoc = new XmlDocument();
      xmlDoc.Load(new XmlNodeReader(node));
      XmlNamespaceManager nmSpcMngr = new XmlNamespaceManager(xmlDoc.NameTable);
      nmSpcMngr.AddNamespace("sp", "http://schemas.microsoft.com/sharepoint/soap/directory/");
     
      String[] names = new String[xmlDoc.SelectNodes("//sp:Groups/sp:Group", nmSpcMngr).Count];
      int groupCount = 0;
      foreach (XmlNode groupName in xmlDoc.SelectNodes("//sp:Groups/sp:Group", nmSpcMngr))
      {
          names[groupCount] = groupName.Attributes["Name"].Value;
          groupCount += 1;
      }
 
 }
catch
 { }