I just tested this on my Pro Plan. If you invite a “Read only” collaborator to an Interface there is no additional billing/charges to your account. Per my previous reply, however, if you invite as a “Commenter” or “Editor” it will add the invitee as ...

Read-only shouldn’t cost more. I will be testing this out soon tho. I did test adding comment collaborators and that does affect billing. Very, unfortunate as this would have made for a perfect customer portal, but not possible with the crazy additio...

This option is now available per the new Connected Apps announcement. That being said, still no info on if you have to pay for collaborators on Interface-Only view