Participants can be invited by the organizers or chosen by a jury. Quebec sculptor Pascal Arch complains of symposia that request maquettes with the application, often without compensation even for returning the model to the artist. Sculptors are in near unanimous agreement in their distaste for competitions or monetary prizes for winners. To make sculpture you need to work in a cooperative environment. Spread the money out between the participants and you'll get good sculptures and happy people.
People do what they always do and rationalize it into a theme.
Public reaction is appreciated by artists accustomed to working alone in their studios. Many people have a hostility for public art because they feel it is imposed on them and they don't know who has done it or why. In a symposium, people ask questions, they walk around it. Sculptors develop little fan clubs of people following their work. They encourage us-they are participating, they respect us for the hard work.