How to maintain a common index for parallel sub processes

I need to develop a process for assigning N volunteers to a project from a prioritized list of people. Each volunteer has a day to accept or reject the assignment. To accomplish this task in the least amount of time we need to spawn N parallel sub processes. The sub process implements a loop that offers the position to the next person on the list until it is accepted.

Implementing this requires the ability to maintain and increment a single index for all N sub processes. This could be done with an external db, but I am wondering if there is a simpler implementation using either a process variable, or a business variable.