How can the Dev. Team help/participate with Product Discovery?
During refinement, a Development Team might assist a Product Owner in ongoing user research for an existing product. Discovery of a new product could happen as a result of interviews, surveys, or perhaps A/B split tests. The important thing to remember is that product discovery is emergent and can happen at any time.
For new products you could try incorporating a Design Sprint. This could be a several day time boxed workshop where things like interviews, A/B testing, sketching, user journeys, storyboards, etc. are conducted. The Development Team holds an important role here not just to ensure their understanding but to discuss the feasibility of the design.
After that, I'd echo what Ian mentioned about product discovery being emergent and happening any time. For large product changes or new features the team could choose to go back and do a more structured session such as a Design Sprint if needed.