Google Maps/Earth are great places to start. I'll use Maps for the highway driving part to sort travel times, motels (if needed), and gas stations. This usually means having multiple browser tabs open, one for the main track and others for segments. The segments are where I punch my start location and try different towns for range/time. When I start to narrow everything down, I'll go back to the main track and start adding destinations and get a better overall of feel for the trip at that point.
Knowing how far you can travel is going to be determined by the members in the trip, and how many there are. Realistically, you want everyone going, driver or not, to have a say in their comfort level. Some people can't do more than 6 hours per day. Kids are a major factor when figuring this out too. I don't have any, but have been on trips where the kids out number the adults. Car sickenss, bio breaks, eating etc. all take time. Try to coordinate bio breaks. When the group gets over 4-5 trucks this time suck can grow exponentially.
Rendezvous points to meet other group members can easily take 20-60 minutes. As a trip leader, I usually give a meeting time 30 minutes before I want the group to be leaving. It's up to you to decide if you want to wait for others past the designated time, or leave on time. En route communications with others in the group is a great way to track your schedule and decide if you want to wait or not.
Google Earth comes in for trail finding and creating the entire track for the GPS file. I'll lay a path for what looks to be the main route, and then add additional routes that look promising (if the main one is blocked, too difficult, closed, turn around/emergency escape points etc.). Some sites/forums have available gpx tracks so you can see if you're on track, haha. This is also where you can start looking into land ownership. You might be crossing a national park/forest, enter BLM land, or cross private property.
At that point I go to that service's website and start checking for permit needs (firewood, or large groups). It's not always easy to make sense, or even find the info you want. About 4 years ago I was planning a Mojave Road trip. I found two different numbers for the number of trucks/people in your group before needing a permit on their site. I talked to a Ranger for clarification. It's good to call well in advance to make sure there are no current closures, or if there are, will they be open when you're going through. Again, using the MR as an example, there was a big washout that closed a large section about 20 miles in. Filing a travel itinerary with a ranger station is a very good idea. For a longer trail like Mojave, you should call the offices at both ends of the trail and give them the same trip info.
Some popular places have local groups with websites that can answer a lot of questions, or give you more timely info. The Rubicon Trail Foundation, Friends of the Mojave, Friends of Black Rock are all examples of locals recognizing the need to get the word out. Some of these trails only stay open through the dedication of these people.
I'm big on safety too. Before every trip I lead, I make sure to collect the following info for each person:
Emergency contact info
Allergies or pertinent medical conditions/info
Their cell number (make sure it's the phone they take on the trip, not a work number)
Details on their vehicle and mods
Recovery equipment if hard conditions are expected
I keep a digital copy of this on my phone and also a printed copy in the truck. I'll let someone else in the group know where that info is just in case. It also becomes a checklist for rendezvous points to make sure you're not forgetting anyone. I've lead trips with 20 trucks and 3 rendezvous points. It's important that you know you have everyone, or have made contact with them so they know what the rest of the group is doing, or where they are. Radio comms are very important for this too, but that's a different subject.
For long or far away trips, I sometimes have a designated point person back home. They have a copy of the itinerary and group list with contact info. They are responsible (when deemed necessary) to make the call to the emergency contact(s). This takes the load off the trip leader to deal with the situation on the trail, while progress is happening back home at the same time.
It's also smart to spend time searching for hospitals, repair shops, dealers for parts, sheriff/ranger stations, auto part stores, etc. But those can be useless if you can't get to them, or figure out how to in the first place. For a long trip, I will find a track for each day to use as an emergency exit. Many times this is the same track you'll be driving, but other times, a road a few miles back will lead to civilization much faster.
Let's see, did I miss anything? Probably, but this is a "quick" brain dump of my approach to trip planning. It might seem like overkill, but one mistake out in the middle of nowhere can be rather costly. A lot of times this info has already been researched, but as a trip leader, it's up to you to be sure the info is current and accurate. One time I set a rendezvous point that we had used for years. It was a gas station, but on this trip it had closed months earlier. It wasn't a big deal as the gas station was in a shopping plaza and there was one across the street, but it drove home the point of making sure your info is solid. The next time we went through that area, the gas station was open again.
