Since your event is more than just a concert it is also a social message, you could for example organize an eMerge 2009 Contest where people can submit 60 second videos on different environmental and social themes that are relevant to youth. This will create media attention and you can premier the Top 10 best videos at the concert. Contact film schools and get "emerging" film directors to participate! The Contest could become your pretext for contacting and involving all the different environmental groups, social groups, and get them to participate in your event. This will get the word out about the event and all these different groups and associations who will premier their videos will bring members. Have the newspapers and radio stations announce the rules of the contest and write a story on it. All this will help fill up the room!