We are travelling to Interlaken on 10 February 2016 and planned to visit Jungfrau on next day 11 February 2016. Need to know validity of Swiss Travel Pass and Return Fares for Group of 15 people from Interlaken - Jungfrau
The additional tickets are CHF 132.75 per person, assuming you'll do the round trip via Wengen and Grindelwald, which is recommended. There is a discount for groups as well, but I am not sure if you can combine that with the Swiss Travel Pass discount. You can ask about that at the rail station in Interlaken.
All information can be found here: myswissalps.com/ jungfraujoch/ tickets and myswissalps.com/ swisstravelpass/ validity.
Please see here for where to best buy your Swiss Travel Passes: myswissalps.com/ swisstravelpass/ price.