How about setting up a HR system?

Admin/moderators can moderate events (date, time, etc). For each events, list down the manpower / resources needed. The system should be open to public. Whoever interested to get involved may sign up as a contributor then based on the vacant slots available, apply for the "jobs". Admin/moderators then choose to approve or not approve.

Besides, contributor may specify their capabilities such as posses own transport. In the scenario of running low of man power on certain area, admin may send notification to the list of contributors base on the the capabilities, informing them which "position" is vacant. It's up to the contributor to pick up the "job" or not.

The problem I foresee will be the content integrity. It's all depend on how soon can the moderator update the time, slots available, etc.

Just a rough idea for now. As for setting up system wise, I'll take care of development, hosting & part of system administration.