Hi teachers,<br/><br/>I'm very much confused with the usage.<br/><br/>According to the dictionary:<br/><br/>Join in: to participate in doing something. <i>I'm glad that you joined in.</i><br/><br/>Join up: to join some organization. <i>Are you going to join up?</i><br/><br/>It was far from clear that the 11 countries that initially joined<b> <i>up</i></b> constituted an "optimal currency area."<br/><br/>1. Is "up" optional in the above sentence.<br/><br/>2. Is "join in" correct?<br/><br/>3. What is the different between "join in" and "join up"?<br/><br/>Thank you.<br/><br/>Tinanam