Press releases are really hard because they're really easy--that is, the information you need to include in them is painfully simple. Figuring out how to get all that in while still writing a catchy, engaging document that will bring in reporters and TV crews and Cokie Roberts and David Letterman is hard--but here, at least, are some guidelines on what you need to include. How to make it cool I still haven't perfected.
First Paragraph--Who/What/Where/When. What's the event? Who will be there, and who's putting it on? Where and when is it? This is all really crucial information, and you want to make it sound appealing. Think of this as the party invitation paragraph.
Second Paragraph--Why. What is the significance of this event? Why is it being held? What will it accomplish (or what do you hope it will accomplish?) You may want to throw in a quotation or two from a group member here, just to kick it up a notch.
Remaining Paragraphs--Deep background. Here's where you might go into the history of this event or of your group, or the biography/credentials of the speaker(s), if this event has any. Any information that seems like it might be interesting or important as background also belongs here. Basically, the rule of thumb is that the less important or interesting a piece of information, the later it should come in the press release. You may also want to bring in more quotations from group members or experts on your issue (the former you can make up; the latter should be authentic).