It's hard for associations to hear. What don't you do by committee? That's what you're about. Getting people in the organization to get together and make decisions for the future good of the membership.
Here's the thing: that might work for membership drives and for your event scheduling, but for your new website, designing by committee is the kiss of death.
I remember recently spending close to 5 hours over the course of several days putting together what was in my opinion a very well thought out layout for a client's new site. When I showed up at their offices they had 7 people at the table to review my work. Within minutes my relevant, strategic design was pulled apart limb from limb until it barely resembled a website at all. With all the convoluted input from the table, what was finally agreed to for the design was lacking in many areas that the committee didn't at the time realize. It also did not match what they had identified as their main goals for the layout: simple and easy to use. Happens all the time.
"Fire the committee. No great website in history has been conceived of by more than three people. Not one. This is a dealbreaker."
Get the idea? As difficult as it may be, as the person responsible for the launch of your future website make sure you keep the input from volunteers and the board to a minimum. If you must have a committee, keep it to 3 people tops.
Keep in mind, too, that at least one of the three people with input on your site will come from your vendor's company. When it's all said and done, you might just end up with a committee of one for your project. Worse things could happen.