I believe the key is using the voucher all in one transaction. You can definitely include others on your reservation, I have done so in the past. You don't necessarily have to keep it to under the voucher value either, you can go above and pay for the difference with your credit card, or even another voucher.

I would be surprised if it expired in 45 days, 90 is pretty standard with Frontier. Were you at one of the out stations? Sounds like this GA wasn't the most well informed.