Is there a way to send mass calendar invites via marketo and have it so the invitees can't see each other?
Can you explain your worry about "see each other"? When you send a Marketo email, no recipient has any idea who the other recipients were. And a calendar invite (whether link to an ICS file or inline attachment, though the latter isn't possible in Marketo) won't show any attendee info you don't want it to.