Hello @Mitcheoj
Thank you for your question. You can only buy seat reservations for the part to Interlaken (but you cannot choose specific seats). To buy such a reservation, go to the purchasable tickets in your chosen connection. At the bottom, you will see the option (5.- per person). To add other passengers, click on the pen on the top of the interface. I hope that helps, else don’t hesitate to ask for more information.
If you travel via Lucerne: The reservation for the Luzern-Interlaken-Express costs more and as I can see, is only possible on a respective website. It’s now 16.-. Depending on the day and time, you should also find seats on this train without a reservation.
On the regional train to Grindelwald, it is not possible to reserve seats.
Have a nice stay!