You sure HDFC MB add on card holders can have/share lounge access within the eligible 12 dom/12 int limit.. Coz I couldn't find it mentioned anywhere on their website.
But I think it will be still considered as Domestic lounge access as it is within India, even though terminal could be International. Any lounge access outside India will be considered as International lounge access.
I have used hdfc marriot bonvoy credit card via digital priority pass app for both primary and add on card members, inside india , outside India and simultaneously and without spend criteria.
Everything is true for this card