Hi Rowdy
That’s right, the Swiss Travel Pass is valid as far as Grindelwald. For Jungfraujoch, you have a discount on the ticket. You will find this information here (More on the topic => Map of validity).
If you are 18 people, you must reserve in advance under the following link.
You will find more information here.
I wish you a nice day.
Kind regards
NicolasK