Hi,
I think you try something like this:
1. Process step
a. Write a service to create a dynamic group with required users.
2. Create dynamic participant step.
a. Assign the step to dynamic group.(http://dev.day.com/docs/en/cq/current/workflows/wf-extending.html#Developing Participant Choosers for Dynamic Participant Step Components)
3. Process Step (At the end of workflow)
a. Delete the dynamic group.
Thanks