Karlie here from the Shopify Guru Team! It sounds like you're needing an app that allows certain customers to access specific pages of your storefront. I have a couple of apps in mind that you can check out from the Shopify App Store.
I would suggest looking into the apps:
Locksmith: Locksmith is an app that allows you to lock specific sections of your store and provide a key to your customers that need access.
Wholesale Lock Manager: This app was intended for wholesale stores, but you can use it with an online retail store as well. This app will allow you to lock specific pages of your storefront with a password.
MagicPass Wholesale: This app can be used on a members-only and wholesale deals, but should also work on retail ecommerce stores. This app would allow you to password protect certain pages of your storefront.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!