Customer Account Scope
The scope of customer accounts can be limited to the website where the account was created, or shared with all websites and stores in the store hierarchy.
To set the scope of customer accounts:
On the Admin sidebar, go to Stores > Settings > Configuration.
In the panel on the left, expand Customers and choose Customer Configuration.
Expand the Account Sharing Options section.
If necessary, clear the User system value checkbox. Then, set Share Customer Accounts to one of the following:
Global Shares customer account information with every website and store in the Magento installation. Per Website Limits customer account information to the website where the account was created.
When complete, click Save Config.