Volunteers can be invited to multiple opportunities. And when one volunteer selects an opportunity other organizations & NVS should be notified that the volunteer just became unavailable. And NVS should assign new volunteers for the vacancies of the other companies.
I think you got this wrong. No organization gets notified about a volunteer until the volunteer accept the opportunity. So this should be corrected. Only the NVS need to be notified.
Volunteers can be invited to multiple opportunities. And when one volunteer selects an opportunity other organizations & NVS should be notified that the volunteer just became unavailable. And NVS should assign new volunteers for the vacancies of the other companies.