Selecting unique Mongo db users?

Question

Python

I recently started getting acquainted with Mongodb, but I already need to select from the list of events people with a unique number who have reached each stage (the stages are written through a string, but at the end of the number, for example "step1" or "step300") but I can't understand how to do it. I will eventually have to go through either the list of unique users and through .group calculate how many each stage has passed and then the dropout will be what I need or be able to find its maximum stage with each unique person. Please tell me how to do it?