Closed User Group

Is anyone here using the 'Invite Only' feature on Substack or Revue?

I want to create a closed user group from my own contacts in a particular vertical but both help desks are coy about being able to do so. i.e Ideally I want to invite my own contacts to view a sample newsletter on either platform with subscribe buttons embedded so they can sign up to receive future ones, but not for other visitors to access accidentally - if that makes sense. In other words I don't want to use the viral method of recruitment that most newsletter authors use where anyone can subscribe.

Both platforms tell me they don't have a closed user group facility as such but has anyone here been able to do a workaround to achieve the same result? e.g Substack also have a 'share a secret draft' link which I've asked them if I can use in conjunction with the 'Invite Only' mode but they haven't replied to my query.

Any advice appreciated.