audience and purpose

I sat in the library for a good forty minutes while waiting on my group for an engineering project to arrive. I decided to take this time to come up with my purpose and audience. As soon as I even began to think.....I hit a mental road block. Every purpose I came up with seemed too vague, too generic, or just not good. So I decided to think about and audience first and then maybe the purpose would be easier to think about if I had someone to write to. My audience is people interested in joining Engineers Without Borders. So the purpose maybe, I thought, should be informing them about the community.....but that is too broad. So more specifically I chose to highlight and show how Engineers Without Borders exists as a service based community. My purpose is to persuade people to become involved by demonstrating how EWB serves both others and its members. Hopefully I can run with this idea, but it may get tweaked along the way.