//get a handle on our sites and web(s)
SPSite Site = new SPSite("<sharepoint url>");
SPWeb Web = Site.OpenWeb();

//get a handle on our current users (if any):
SPUserCollection users = Web.AllUsers;

//add the new user (see my snippet on getting users from Active Directory, which comes in handy here)
users.Add("<login name>", "<email address>", "<name>", "<notes>");

//no need to do an .update()
//you can now return this user as an SPUser:
SPUser newUser = users["<login name>"];

Add user to SharePoint

directory, user, sharepoint

C#