参与 (cān yù) is more formal than 参加 (cān jiā) and often implies active involvement in a process, discussion, or project (e.g., 参与决策 'participate in decision-making'). 参加 is the everyday word for joining an activity, group, or event (e.g., 参加比赛 'join a competition'). Use 参与 when the focus is on contributing or being part of something, not just showing up.