Hi,
I checked your requirements and I can create an app in WIX using js to read & update the google spreadsheet. I believe the app will act like an login system to the site.
Pls PM me to discuss further details.
I have 4 years of front-end devlopment using HTML, CSS, JS, jQuery, Bootstrap and AngualarJS. I am proficient programmer in asp.net and php also. I can assure you that I will provide quality work in given time period without any issues. The final code will be delivered with comments and documentation.
Looking forward to work with you,
Kasi