Participatory systems have been widely adopted in citizen science, environmental monitoring, urban governance, and public collaborative decision-making. Traditional usability theory focuses on individual task performance and user satisfaction, which cannot adequately explain or support voluntary collective participation, participant recruitment, and long-term engagement. To address this gap, this study introduces the new concept participatability and develops a dedicated assessment framework for participatory systems. Based on a systematic review of usability criteria and the unique socio-technical features of participatory systems, this study defines five core evaluation dimensions: salience, adaptability, congruence, privacy safeguarding, and interactive engagement. Two complementary case studies, including a mature citizen science platform and a newly developed campus participatory planning system, are conducted to validate the framework. Empirical results show that participatability is significantly associated with user acceptance, participation willingness, data contribution quality, and long-term system sustainability. Users in collective participation scenarios prioritize participatability over conventional usability. This study provides a theoretically sound and practically applicable framework for understanding, evaluating, and designing participatory systems. The proposed concept and criteria address critical limitations of existing theories and offer practical guidance for system developers and practitioners to improve participation effectiveness.