N.A. (before July 2018)
6. August 2016
5 (during the last 30 days)
Short answer: yes. As for where...it's hard to say in advance, as the situation changes frequently. Please prep by reading http://bit.ly/howtovolunteeringreece. You can safely commit to buying a ticket to either Athens or Thessaloniki, then adding on tickets to specific destinations later.
Reach out to various organizations, but understand that administrative support is limited, and you may not get a response immediately if your planned trip date is more than a few weeks away. It's stressful, we know...but trust that plans will fall into place and you will be useful in the end.