Right here, OP!
No guide needed. You can do all this yourself, and have a much, much more enjoyable experience. I'll just briefly touch on a few things of which you should be aware, and then you'll have a bit more info in order to help you possibly ease your mind into doing this yourself :) Impress your friends! :D
I'm going to rant on about some key steps you might want to take should you decide to conquer this endeavor yourself, OP. Slight wall of text incoming. Don't get overwhelmed, just take it one step at a time.
Get yourselves familiar with Rider Planet USA and All Trails ATV. Find out your general camping location, or where you plan to reside while your out there, and then find some trails within the vicinity.
Okay, so you're going to want to phone this in to as many places near where you plan to be as possible. You want to feel out the rental places, and feel out their attitudes. Fine one that gives you a good vibe, and one that doesn't mind you addressing all of your concerns over the phone. Concerns worth mentioning are: operator on-trail maintenance (do they permit you changing the oil, filter, spark plugs, et-al, out on the trail should the need arise?), and if they know local rescue off-road rescue services, etc. There are more, but those two are pretty important, especially the O/R rescue services (after all, what happens if one, or hell even two, of the ATV's break down 25 miles into a trail?) You'll also want to ask if they change the oil, oil filter, spark plugs, and air filter before every rental (they should); gauge their reaction here, and it will tell you multitudes about how much they care about your enjoyment rather than the profit.
Make sure you find out what types of ATV's you will be renting when the time comes. You'll want to, hopefully, rent all four of the same exact make and model. This will make any maintenance you may have to do while riding easier. Look up the information about the ATV's, and look up things like how to perform oil changes, oil filter changes, air filter cleansing, spark plug changes, etc. Youtube has a wealth of information about all this; it's not as daunting a task as it seems :) Preparation is great!
From what you're describing, your party would like to ride some long, beautiful trails; really get to know the land. You're going to want to rent some high quality utility ATV's. You want comfort, because rides like this are meant to be experienced, not pullin' flat-out (wide open throttle). Anyways, I would suggest finding yourselves some Honda Fourtrax Foremans that you can rent. You can't go wrong with a 4x4 quad that won't give you trouble, and one that's as comfortable as a Cadillac to ride (I'm not joking). Then again, I'm Honda-biased for reliability, and comfort. I'm not a huge fan of tearing up the trail and going fast. I like low and slow, rock-crawling/mudding. But, you can always opt for some Yamaha Grizzly's, or some Polaris Sportsman 850's; these have much larger engines than the Honda's, go much faster, and ride very decent. They'll get you where you want, and have storage capacity to bring tools and backup supplies. Which brings us to...
This is purely based on the rules and regulations of the rental shop with which you do business. Some places permit operator on-trail maintenance, and lots of places do not. Ask them, and they will tell you.
You'll definitely want to make sure you have backup oil for each ride, as well as oil filters, spark plugs, and possibly even air filters. Also, you'll want tools that can do the changes for each vehicle, when you're out in the boonies. The last thing you want is to, say, flood your engine and then have something simple go out like the spark plug. Oil, oil filter, and plugs; get a basic set of tools, like a ratcheting screwdriver that has a lot of drivers in the kit for philips, flat, hex, torx, etc. If you can, you might even spring to bring some turpentine to your camp for cleaning your air filters should they get clogged with mud or what have you, and filter oil. Planning ahead is key, and as you've stated you're not going until next year, which means you have plenty of time to research :)
I would also recommend that you each get yourselves a two-way radio. Here's a three-pack of Motorola T260TP's on Amazon for ~$70.00. This will make communicating with each other easier if you get turned around, or, you know, lost lol. Accidents happen, so always be prepared.
For goodness sake, get yourselves a GPS with some good topo maps. A Garmin eTrex 30x is simple enough, but the GPSMAP 64st will keep you better positioned with its higher accuracy.
Planning these trips yourselves is super easy, and luckily you're planning way ahead of time, which makes it the perfect time to learn how to do this yourself :) And, next time, it'll come even easier. With time, you'll be wondering why you didn't start earlier :D
PS: Sorry that I rambled on, I just couldn't help myself. I hope this helps, whatever you decide! Good luck!