My travel group of 10 is doing Norway-in-a-Nutshell on July 20. We've booked the components on our own instead of having a travel company sell us a package of tickets, and it's worked fine. We've had the Naeroyfjord cruise tickets for a few months, and got the Flamsbana tickets a couple weeks ago, and over the weekend, I booked tickets from Oslo to Myrdal. We want to get an early start to the day and booked the train that leaves Oslo at 6:28 a.m. On the nsb website, I couldn't get all 10 rail tickets in one booking. It only allowed 7. I knew there were ways for me to work with the rail company to book a larger group, but it didn't bother me to do a booking for 7 (listing myself as primary passenger) and then a booking for 3 (listing my husband as primary passenger). Some great minipris tickets were showing up and I didn't want to put this off until after the weekend. At the end of the booking for 7, I was allowed to pick seats and did that. When I did the booking for the remaining 3 people, a minute later, the seat selection page was showing me a different car, and there was no apparent way to change it to the other car that the other 7 are in. So we're in two different cars, which is fine, though now of course I wish I'd split the group as 6 and 4 instead of 7 and 3 so we don't have two people sitting alone. Maybe the 6:28 train never sells out and once we leave Oslo, members of our group will be able to freely move between cars and pick different seats? Can anyone comment on that?
My hunch is that since it allowed you to select seats, and they're the cheap fare, you're not going to be able to move around once you get on the train. If you're going to be in Oslo the day before, why not pop into the train station and ask if they can move the three seats to the same car? That's a popular time of year, so it is possible the train will be pretty full. Check the fare details also, there may be a fee to change seats. Regardless, it's a beautiful trip, and it's only a few hours that your group will be separated.
You can normally walk through the train with no problem and use any unreserved seat. I don't know whether NSB marks which seats are reserved (e.g., if not every boards right in Oslo). Better to be in your reserved place until the conductor checks tickets, and then you can ask how full the next car is.
I think you should't worry much about that, you have quite an early train, and as was said above, you will be only separated for a few hours only. As far as I remember Flamsbana train itself has free seating, so for Myrdal - Flam route you're most likely will be sitting together.