Multiple Instances With a Priori Runtime Knowledge

I’m modeling a workflow that has a step that could generate multiple instance of the folowing steps (assigned dynamically to different users of the same role) and I was wondering how can I merge then these multiple instance because all of them need to be merge in only one step after and merge pipeline data.
This workflow pattern is explained here

I figure out how can I generate the instance But I cannot figure out how can I merge all of them,

Thanks in advance, any ideas are welcome.