SharePoint Server 2010/2013 error while adding Forms Authentication based user - Microsoft.SharePoint.Client.ServerException: The user does not exist or is not unique.

Hi All,

I am trying to add a user to ProjectServer 2013, the site is setup to use forms based authentication so no domain name is being specified in the user name. I get the error -

Microsoft.SharePoint.Client.ServerException: The user does not exist or is not unique.

I have used Remote Authentication in SharePoint Online Using the Client Object Model. Please suggest

code -

UserCreationInformation userCreationInfo = new UserCreationInformation();
                        userCreationInfo.Email = USER1@TEST.COM;
                        userCreationInfo.LoginName = USER1;
                        userCreationInfo.Title = USER1;
                        User currentUser = clientContext.Web.CurrentUser;
                        User user = toBeAddedGroup.Users.Add(userCreationInfo);

                        UserCollection collUser = toBeAddedGroup.Users;
                        collUser.AddUser(user);

                        //Loading User, group and executequery.
                        clientContext.Load(user);
                        clientContext.Load(toBeAddedGroup);
                        clientContext.ExecuteQuery();