They come from within the community with a certain desire. The desire to serve. They fill out applications with their respecitve governments and then they hope that they get picked. Some applicants are ambitious enough to get letters of support from community leaders. Others work through the elected officials who will make the appointment.
Community boards require commitment.
There is serious business to be conducted. These are not boards for people who are looking to fill holes in their resumes, to create stepping stones to other boards and possibly elected office. It sounds simple enough to get involved. Given the population this county, there should be people beating down the doors to get on these boards. But, it doesn't always happen that way.