At worst, you could certainly rent a car (or two) for the day and just drive there. If I were alone, I might consider public transportation but probably not with a big group.
Once you get to the park - e.g. drive to Skradin and park - it's easy to self-tour. Buy park tickets at Skradin, get on the hourly boat that goes upriver to the bit Skradinski Buk waterfall (beautiful), walk up past the old power plant etc. and across the river and back and even swim below the falls if you go in the warm weather, then take the boat back to Skradin. There is more of the park than that - see the park's website. There are some Roman ruins and another big waterfall for example, but I didn't see anything but Skradinski Buk as well as do the boat ride.
I'm guessing you can find options for an organized tour - have someone pick you up at the port and drive everyone to the park, but as I said you really don't need a guide just for Skadinski Buk; it might be nice to have a driver shuttling you to the other parts of the park if you want to see them.
If you don't want to spend all day in the park, there are other things to see - like the nice towns of Sibenik (close to Krka but on the ocean) and Trogir, near Split.