sharepoint Working with Managed Client Side Object Model (CSOM) Groups. Adding a user to a SharePoint group
Example
ClientContext clientContext = new ClientContext("http://MyServer/sites/MySiteCollection ");
GroupCollection collGroup = clientContext.Web.SiteGroups;
Group oGroup = collGroup.GetById(6);
UserCreationInformation userCreationInfo = new UserCreationInformation();
userCreationInfo.Email = "[email protected]";
userCreationInfo.LoginName = @"DOMAIN\alias";
userCreationInfo.Title = "John";
User oUser = oGroup.Users.Add(userCreationInfo);
clientContext.ExecuteQuery();
