You don't have to tell anyone their driving stinks, just say you know where you're going, you have directions, so if you lose each other along the way, don't worry.
I really don't like convoys because you can so easily lose people along the way or get lost. I would rather have an address and phone number and some directions, and provide these things to to other people. This way, if you lose each other, everyone still knows where to go and you don't have to hang back for the turtles or try to keep up with the crazy people that are weaving in and out of traffic and blowing through yellows. If you find yourself hopelessly lost, you have a phone number or two or three to work with.
"Don't worry about watching for me, I have the directions, I know where we're going. If we lose each other, I'm on my way. If there's a problem I'll call you, and you have my number if you need to call me, right? Let my write it down just in case."
Maybe you can agree to meet at someplace in the middle. Depending on the length of travel, people might like to use the restroom, stretch their legs, get something to eat, so you can try to connect in that place, count heads, and then head back out on your journey.
I always, always go after directions because I can be quiet the white knuckle driver on unfamiliar roads and freeways, especially mountainy and curvy roads. I'm usually the turtle. It is unsafe to try to keep track, and I have found myself with absolutely no idea where I'm supposed to go because I lost my leader somewhere because he's driving like a maniac. It's just easier to to have an address. You can try to convoy, but understand you might lose each other. With cell phones, communication shouldn't be a huge issue.