The NPCs who will build the station are selected from NPCs of the type Builder who are at your location, they don't need to be in your party.
However when I set to begin construction on a station I was not allowed to begin until the NPCs were in my party. I was assigned as their supervisor and was in the same room as them. I had 25 to start, supposedly the max you can put on a station. Before I added them to my party, the station construction menu would display that there were no builders present. Then I add the max of 12 to my party and try again. Bam, those 12 only show up.
Is this a bug or .. what? I now have 13 extra builders, increased construction costs due to the number of builders used and longer to wait before I can even store these builders somewhere for the client. Slightly frustrating, can someone help out with this please.
Be mindful of the future, but not at the expense of the present.