JKim, thanks for asking about the situation. There are lots of groups who have an extra space or two, just like you, for the same reasons. It's just that not many people write about the situation.
The easiest (and kindest) thing to do is as wazzu wrote: log into recreation.gov, and change the number of people on your permit. Those unused slots will be made available at a random time within 24 hours of their release. So someone will most likely snap them up.
If you held onto the slots, and offered them to people, you are still the group leader and responsible for the others. I'd sure not want to be in charge of a couple of unknowns.
If you did not return them via recreation.gov, but instead just walked into the Visitor Center and picked up two and dropped two, then those would be made available to people walking in.
Please, go online and release them early, so others can be sure of getting them, and can prepare for their own hike.
