This is going to be a very short blog as I am going to just reference my previous blogs to achieve a solution for this.
I have talked about Customer Journey entity in dynamics in a recent post here: The ‘Mysterious’ Customer Journey Entity In Dynamics 365 Marketing.
By now, we know that it’s not easy or straight forward to get any details from a customer journey from the back-end.
In a customer journey, customers are not directly added but rather through a “Marketing Segment”. Now we have seen in the blog above on how to get segment details used in the journey. Once we have the segment Id, the next step is to fetch the customers from the segment and for that I have another blog explaining it: Dynamics 365 For Marketing – Retrieve Contacts From A Segment
By doing the above mentioned, you should be able to get all customer that are part of a journey.
I hope this helps.