select ..., p.event_id is not null from events as e left join participations p on p as.user_id = ? and e.id = p.event_id
select ..., exists(select null from participations as p on p.user_id = ? and e.id = p.event_id) from events as e
NoinoNo. It is sufficient to determine the user ID in the "Members", the subquery, for example.