A way they could learn on the job would be from one another, through Moodle. Set up a course for the volunteers where they can exchange information about how they answered specific questions that arose during their job. They could use wikis to develop manuals on specific topics that are frequently asked about. You can train them in the basics, but in the long run it is the volunteers themselves who will become the experts on how to do their job.
This would work even better if the volunteers have computers in front of them while working, the Moodle could become a resource for them to do their job.
General developer forum
Continuing Education Project
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