Intertwining users' engagement with the development of new products is becoming increasingly important for improving the products' performance. Therefore, a method to model the dynamics of collective user engagement is essential. This paper proposes a method to model the dynamics of collective user engagement in new product development using agent-based simulation. The model integrates an indirect engagement process – in which users exchange information and knowledge, draw from past experiences, and psychologically invest in the product – with a direct engagement process – in which users interact with the product and the firm to contribute. We demonstrate how the model can be calibrated using a consumer survey. Our findings reveal an overall increase in user engagement, characterised by nonlinear growth during the idea and design stages, followed by a relatively steady increase during the test stage. Moreover, our study identifies the significance of various factors, such as social network parameters, past experience, product connection, and match degree, in shaping user engagement in new product development. This research provides a systematic approach to model user engagement within the context of new product development. It also offers practical implications that can guide management decisions in effectively engaging with users during new product development.