An executive's calendar is defended by default. The question they are answering is not "is this interesting" but "do I owe this person a reply". A branded invitation from an unfamiliar sender fails that test before the subject line is finished.
So the first job is to find the warmest existing tie to each person you want in the room, and have that person ask. Across a company of any size, the strongest relationship with a given executive is usually held by someone outside the sales team - a founder, an engineer who worked with them, the CSM on a neighbouring account.
The second question is who else will be there. Executives come for peers.
That means the sequence matters: confirm three or four credible names first, then use them. "Four other heads of revenue from companies your size, including two you'd recognise" is a reason. "Join us for an evening of insights" is not.
This is why the first invitations should go to champions and existing customers rather than to targets. They are the proof that makes the rest of the list accept.
A roundtable topic should be a problem the attendees are currently failing to solve, phrased the way they would phrase it internally. Three tests:
Sending every invitation from one person - usually a field marketer - because it is operationally simpler. It converts the whole exercise back into a campaign, and the acceptance rate reverts to campaign numbers.
Distributing the asks across the colleagues who actually hold the relationships is more work to coordinate, which is the problem Orbb's Events Agent exists to remove: it assigns each invite to the person with the warmest tie and tracks who has asked whom.