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.