Once again, the folks at http://architexts.us tell it like it is.

For many employees in a design office, lunchtime educational programs are one of the few perks they get. Use their hunger (for education and for nourishment) to your advantage.

They may come for the food, but make sure they leave with something more than a full belly. (I mean useful information, not just leftovers.)

Most people that make the effort to attend a professional society meeting are highly motivated. This makes them a good prospect for what you have to teach.