Hello Faubeassif91
That’s a lovely trip!
Please kindly note, the Glacier Express is not operated by SBB yet by RhB and Matterhorn Gotthard railways.
You cannot reserve seats for such a large group by yourself via web. I strongly recommend contacting the RhB rail service. The staff there is very helpful in reserving seats for your group.
Please see the contact details here: https://www.glacierexpress.ch/en/travel-planning/group-travel/
Get in touch with them soon, depending on the season, the Glacier Express is sold out very quickly.
The Swiss Travel Pass is of course accepted, and you will only pay for the seat reservation.
Kind regards,
Roland