If your friends have any experience at all, even just a little, snorkeling the shallow reefs will not be a problem. Most ops will require a safety vest and discourage freediving if your people are inexperienced. So, all they need to know is how to blow out their snorkel, unless they have a dry snorkel which eliminates even that issue. They can practice in the pool where you are staying. Or, if you want, the water is certainly clear enough close to shore to snorkel for a few minutes just to get used to it. I don't think there is a need for practice trips before hitting the reef unless they are really uncomfortable. Near shore vis is usually 20 to 50' unless it has been choppy, which stirs up the bottom. You can see interesting things close to shore, if you are lucky, but the real magic of the Keys is the reefs and that is where all of you want to be. In other words, don't pass up a reef trip to make a "practice" shore dive.
Carysfort has a large area of coral that is between 3 and 10 feet deep near the light tower, and the tower itself has excellent snorkelling, 8-15 feet deep, lots of fish, and sometimes lots of barracuda hanging out. Carysfort is right next to the Gulf Stream, and visibility can typically be 80' plus. Basically crystal clear. A really "bad day" is 30' vis. This is a fine snorkel even for beginners who have vests and are not freediving. As always, conditions need to be good (March can be windy and choppy at times) and the dive operator will let you know if things are right. I am a lifelong scuba diver but I never pass up a chance to snorkel Carysfort when I get it. In other words, it will be a treat for you as well, even if you just snorkel.
At carysfort, there also 15-30 foot areas close by for scuba diving, and a deeper reef at 60 feet. I know for sure that Quiesence will take both snorkelers and divers on the same boat and will go to Carysfort on request, especially if you have several divers together that request it. Quiesence runs 6-pack boats which is really great, and if you have multiple divers that want a particular spot they will accommodate you. I also know that Silent World does Carysfort, but don't know if they take divers and snorkelers together. They probably do, but call them and check. There are also other shops on North Key Largo that might go there.
The reason you don't hear about Carysfort is that it is visited far less often than the other, more southerly reefs. It is a longer run than most of the other reefs, and so many shops do not go there to save fuel. Silent World has a good description of carysfort on its website, more oriented to scuba diving. I believe that people who go there will support my view that it might be the best diving (and especially snorkelling) in Pennekamp Park. That being said, it is hard to go wrong with most of the snorkeling reefs in Pennekamp. Carysfort is a special treat, but certainly not the only game in town. The ops I mentioned go to other reefs that have very shallow areas as well. Also, there is a dive/snorkel concession right in Pennekamp that will take people, even total beginners, snorkelling on some of the closer reefs which are very calm unless the weather is just terrible.
I know this is a lot of information for what would be a single morning's outing, but Carysfort can be worth the effort.
If you are comfortable in boats, another option is to rent a boat for a full day. That way you can hit several reefs and mix up snorkelling and diving. If you are interested in a dive buddy to share the cost, keep in touch with me through the forum, as my wife and I would be glad to join you and I can pilot the boat. We usually do the boat rental thing several times a year. Cost for a 25' center console quality rig that can take 6 people is about $450 to 500 including fuel from 8:00 to 5:00, not bad if split between 2 or 3 groups.