What is the method of using sand to allow attachments to stick on veneer surface? Can it damage the porcelain veneer?

While this process is very "gentle", one could argue that it IS damage.  When done, the veneer can be polished smooth again.  All of this is very minor.

Hi Rzayan,The etching creates small grooves on the surface of the tooth that allows your Invisalign attachments to grip onto. Since they were sliding before, the etching is necessary for the Invisalign to work it's magic properly. When all is said and done, your veneers can be polished smooth again.

Like somebody already mentioned, its called micro-etching and it roughens the surface of the veneer.  Ideally, those teeth could have been excluded and no attachments placed, but your dentist should be about to polish the surface at the end of treatment.