SharePoint - Unable to Add More Members to Site

I notice that the site is a private channel site. According to this Microsoft article,

When SharePoint sites and Teams are integrated, certain setting and classifications need to be managed in the settings in Microsoft Teams. You notice that when you navigate to Settings in SharePoint site, certain options aren't editable, like Permissions. Channel sites display site permissions in the SharePoint site permissions panel in read-only mode. Site owners can access and update site sharing or guest expiration options in the panel, but they need to go to Microsoft Teams to manage site permissions.

It's by designed that you cannot add members for private channel site in the SharePoint. And the site does not display the number of members next to the "Following" icon by default.

Log in Microsoft Teams with your owner account -> Click Teams in the left navigation -> Find the private channel -> Click three dots in the right hand -> Add members.

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