You should have a 'club' icon near your 'quests' and 'free' icons. You can click on it and scroll through the clubs and choose to join. :)

It will keep taking away a card from your draw pile the longer it is on your board. Try and get rid of it as quickly as you can.

It will give you double red stars when you are playing the game. So normally you can get up to 3 per level. If you have that activated (which lasts for a couple of hours), and you get 3 red stars it will give you 6. Your red stars go towards your total for the leader board, in which you can earn additional coins at the end of each week. :)

The longer the wave card sits the more cards it will take away so try and get rid of it as quickly as possible.