Complex sentence: The club decides to play a game where each member tries to make Haruhi happiest within an hour and one thousand yen and head off taking Mei with them and leaving Haruhi by herself.
Split into simpler sentences: The club decides to play a game where each member tries to make Haruhi happiest within an hour and one thousand yen. Most of the club head off, taking Mei with them and leaving Haruhi by herself.