Правилният синтаксис би бил:
Aggregation aggregation = newAggregation(
match(Criteria.where("_id").in(ids)),
unwind("$attendees"),
match(Criteria.where("attendees.reminded").is(false)),
project("_id","attendees.contact.email"),
group("_id").push("attendees.contact.email").as("emails")
);
Справка:http://docs. spring.io/spring-data/mongodb/docs/current/reference/html/#mongo.group