This paper proposes an optimal method for selecting prospective participants to enhance the representativeness of the Citizens' Assembly, a participatory decision-making body. To address the problem of member bias resulting from low participation in the Citizens' Assembly, we highlight the limitations of existing prospective participant selection methods and propose an algorithm that utilizes machine learning techniques to predict participant attrition and maximize representativeness. Theoretical analysis provides guarantees against sample complexity and attrition prediction errors. Experimental results using real-world data demonstrate that this method significantly improves representativeness while reducing the number of prospective participants compared to existing methods.