Please do not confuse the Half-Fare travelcard with the group ticket.
The Half-Fare Card is personal and each adult should decide for themselves whether or not to have a Half-Fare Card. The Half-Fare Card gives one person the right to buy tickets at a reduced price.
The group ticket is for that one trip you mentioned. Even if no one would have a Half-Fare Card, we would still sell you a group ticket because of the number of people departing together. Also, the conditions of sale for group tickets are always the same, regardless if someone has the Half-Fare Card or not: 30% discount on the normal price, free seat reservation, etc.
It may be a bit difficult to explain everything here. Please have another look at the links I mentioned in my last post, we have als FAQ, and if you still have questions, please call us.
If you call 0041 51 225 78 00, you will be charged the local cost of the call.
I wish you the best. EllenvS.