When I hear "We are looking for volunteers to help out at our annual event" I'm actually hearing "We've got some work for you that will take an indeterminate amount of time ... and by the way there is a good chance you won't enjoy it".
Consider rephrasing your request. Don't ask for volunteers. Instead welcome people to "Get Involved". Then let them know:
- The positive benefits for them (learning new skills for the resume, expanding their social or business networks, etc);
- the type of work involved;
- the time committment (date, time and duration); and
- who to contact.
"Are you interested in meeting some of the most successful people in our industry? We're looking for someone to attend our annual networking event as our Member Representative. Your role will be to ensure all our new and prospective members have a great time. This is a great opportunity to expand your networks in a social environment. We are looking for someone who can spare 2 hours from 6.00 on DATE. To take advantage of this opportunity contact PERSON via EMAIL or PHONE"