The first thing we do at Digital Methods Corporation when embarking on a new project is to ask some very simple questions. Even though often it seems that the answer is obvious, it is important to start with these questions to hear in the client's own words thier business vision and the challenges they face..

1. Who is the audience? - This is not a drill down into audience segmentation but a broader question about the "users" of the system.  In this case it was Members, Non-members, and site administrators.
2. How does the business run?  In other words, what is the business model? In the case of BMA it is a combination of membership fees, and money generated by events, contests and sponsorship.

These two questions cover a lot of ground and create a mutual understanding between the client and the agency regarding the project.  There are much deeper discovery and elicitation activities ahead, but with these two questions, we can establish a number of different requirements and goals for the project.
I've attached a draft of a simple requirements document here to show some of what can be established with some simple questions.  Again, it is a simple document that can be greatly added to, depending on the style of project methodology you choose.
If you are going with a standard "Waterfall" or Rational Unified Process methodology, there will be a great emphasis on this documentation up front.  If, as was the case with BMA, we went for a more "agile" approach, we set up an online tool (Redmine) and created a collaborative workspace for developing and discussing the strategy, features, schedules and tasks required to complete the project.
To quickly summarize the findings from our first meeting and the answers to the two questions above:

1. Three User types:
   1. BMA Chapter Members
   2. Non-members
   3. BMA site administrators
2. BMA is a membership driven group that is not for profit.  BMA generates funds for the operation of the chapter through the following activities:
   1. Membership fees - a portion of national membership fees is passed down to the BMA local chapters
   2. Event fees - BMA operates monthly luncheons, and breakfast events, additionally they have several annual events (BizBash and Tower Awards) that also generate operational funds for the BMA.
   3. Sponsorship

From these answers we can make a small leap of logic to make the following statements:

1. The BMA site must aid in the generation of operational funds. 

From this we can then draw the following conclusions:

1. The BMA  website must provide the ability for non-members to become members
2. The BMA website must help publicise and generate interest and action in attending events
3. The BMA website must provide visibility and measurable value to its sponsors

For the user strategy, we quickly came up with several clarifying statements that helped us understand the broad usage scenarios that we would have to accomodate.
Non-members will visit the site:

1. seeking  information and vallidation about  the value of membership

Members and non-members will come to the site:

1. seeking information on event upcoming events, pricing and attendance
2. to assess the stature of the chapter membership
3. to assess the sature of the chapter board

The Site must:

1. have features that are only available to members
2. communicate the benefits of membership and the available features to non-members
3. offer member features that re-affirm and validate the cost of membership
4. collect data about user activity, site traffic, and business goals
5. make the lives of the site administrators easier
6. communicate the values and brand of the BMA national organization as well as the values and brand of the local chapter.

Each of the statements above is a definitive "requirement" and one that can be now be broken down into more detailed requirements and then into project features. 
One of the main issues I see in community project is the desire by both agency an client to dive too deeply into "solutions" before they understand the requirement that the solution is addressing.  At this stage of the requirements process, we have some very broad requirements, and practically any set of content management systems, custom applications, or static web pages could address these in the simplest sense.
The value of a good web development process is to allow a diverse team of experts to communicate their expertise in a common language without compromising the sophistication of their specific discipline.  Marketers, web geeks, operations people, IT staff and consultants all speak very specific and different professional languages out of necessity.  The value of requirements is to strip out jargon, value statements, marketing speak or technical details and create simple statements that any member of the team can understand and create a specific activity that allows them to contribute to the solution.
Notice we haven't spoken about the software solution at this point, as none of the above answers have in any way limited our pool of possible solutions.  The only technological mandatory is that the site be available on the web.
We also specifically haven't mentioned creative development beyond stating that the site must be "on brand".