Bulk Account Inviter
You can use the Bulk Account Inviter app to invite your customers to activate their accounts for your online store. This app is useful after you have imported customer accounts from another platform or from another Shopify store.
Create a customer group that contains all the customers that you want to invite. For more information, refer to Create customer groups.
From your Shopify admin, go to Apps and open the Bulk Account Inviter app.
In the Customer group drop-down menu, choose the customer group that you want to invite.
Click Create Email. By default, your invites are generated from your Account invite notification template. If you want to change the subject or the content of these emails, then edit the Subject and Custom message fields.
Choose an email address from the email drop-down list. This is the email address that your customers see when they receive the invitation.
Click Review and then click Send email.
The email invitations are sent. Depending upon the number of customers that you invite, it can take a while for all the invitations to be sent.