This will vary based on the company in question, and the product that is being developed. Basically, you want professionals whose positions are needed in addressing the challenge being faced. This includes the personnel building the product, those who engage the customers and are thus well placed to know their preferences and pain-points, business area experts, as well as the decision-makers in the company. As such, participants in the workshop can include the UX designers, marketing crew, executive sponsors, managers, and even actual customers. This is to give you different perspectives on the product, its features, consumer needs and market reception.