Hello TravelwithUs
You actually do not buy a ticket for the Glacier Express, but a seat reservation. Tickets cannot sell out, in contrary to the mandatory seat reservations.
I suggest you to contact the Glacier Express Customer Service. They usually block seats for groups in advance which they can assign to individual travellers in case of no-shows or smaller groups than actually expected.
You can do the same trip using regular trains. The downside of it, you have to change trains multiple times and there is no dining possibility. Of course, you could do as the Swiss do: grab a bread, some dry meat or cheese, some veggies etc at Coop or any other grocery store and then have a picnic on the train while the same landscape is passing by as you would see when using the Glacier Express.
Kind regards,
Roland