I think I have worked out how to automatically log in a visitor as a registered user:

        private bool LoginVisitor()
        {
           
            //Try to validate user
            string ipAddress = "";
            if (Request.UserHostAddress != null)
            {
                ipAddress = Request.UserHostAddress;
            }
            UserLoginStatus loginStatus = new UserLoginStatus();
            UserInfo userInfo = UserController.ValidateUser(PortalId, loginUsername, loginPassword, loginPassword, PortalSettings.PortalName, ipAddress, ref loginStatus);

            if (userInfo == null)
            {
                //login failed
                return false;
            }
            else
            {
                //user is logged in
                UserInfo objUser = UserController.GetUserByName(PortalSettings.PortalId, loginUsername, false);
                UserController.UserLogin(PortalId, objUser, PortalSettings.PortalName, ipAddress, true);
                return true;
            }
        }