I have a WordPress multisite with a main site on it. From the multisite settings, registration of users is enabled.

When a user registers and confirms through email, they are added to the multisite but not to the main site. They have to be manually added to the main site.

How can I make it so that users who register on the multisite get automatically added to the main site also?

Technical details: It’s a subdomain-based multisite, running WordPress version 5.3. The default user role in the main site settings is set to subscriber as defined in Network Admin => Sites => main site settings.