First, hunting is not allowed inside the park:
This is a pretty long answer, but I think that how you speak to a group of people is just as important as the information that you give them.
I’m sure there are many different ways to address such a group as you described (people in a hunting club), but here are a few things I would keep in mind.
1) Know the problem. Are people taking the wrong kind of animal because they honestly cannot tell the species or sexes apart, or do they just not care? These are two very different problems that require different approaches.
2) Know your audience. If you are at a hunting club then most of the people there are probably experienced enough to tell species apart and already know most of the regulations. Avid sportsmen should already know the basics, in the same way that a driver knows the basic rules of the road. That doesn't mean that there aren't jerks out there willing to ignore what's ethical (just like you get road-rage drivers), but they are rare.
3) Seriously, know your audience. I once had a raptor rehabilitation presenter come to one of my wildlife classes. Her prepared talk was obviously the one she was used to giving to the general public, and some of the simplistic questions she was asking us sounded almost condescending, especially since most of the students in the class could give her the scientific name of every bird she was talking about. The hunters and fishers at the club are not going to listen to someone who talks down to them.
4) Establish a good relationship. Ask them what they’ve gotten so far and congratulate them. If the ranger hunts or fishes (most do) then they might describe good areas to do so or where they’ve had success. Especially if the problem you have is people willfully ignoring regulations, it’s vital that you are on good terms with the people you’re trying to educate.
5) Use photos. If the problem is an inability to ID species, then simple, cheap ID books might be a good thing to distribute. This is not likely in a hunting club, but tourists coming from outside the state who only occasionally fish might have a problem. Also, if there are unique laws in Yellowstone that the sportsmen don’t know, simply informing them of those special regulations is usually good enough. Remember, only a small portion of hunters and fishers willfully poach game.
If people really just don’t care then here is how I might approach the situation (note, I am NOT a ranger. This is one way to go about this, not necessarily the best way). I’m going to use fishing as an example since it’s legal inside the park. If a particular species is off limits I would have a simple flyer with a photo of the problem species and I would appeal to the sportsmen to “help” the park maintain that species by releasing it if they catch it. I would mention that, as someone who loves to fish, I personally do not want to see popular fishing areas closed. However, if the population of that fish species drops too low due to overharvest the park will be forced to place fishing restrictions in areas that it lives. There are certain federal laws concerning threatened species that might require closure of an area, I just don’t know them off the top of my head.
I hope this was helpful.