I wrote a (quite complicated maybe?) guide to finding friends as a reply on page 11 for nothing then.. 
If I had spotted your post, I wouldn't have written mine 
Just messing..
Here it is again, in case I've added something useful..
You can cruise the island together, as long as you are on the same server. Here's what you need to do (sorry if it reads like an idiots guide!):
1. Get in a private chat room through the guide button, so at least you can speak while you're trying to meet up.
2. One of you will need to just sit on the road and wait for the other person to show up. Don't wait on the map screen though, just sit in your car on the side of the road so that the other person can find you easily.
2. The other person will need to go into the gps map and press RB or LB to change the information on the gps screen to players/people. press X to open the filters and change it to friends (at the bottom).
3. Highlight the friend on the gps screen using the sticks/d-pad and press A (not B ), it should say 'go to' or similar. You'll zoom down to where they are.
4. Wait a few seconds for the server to connect (literally a couple of seconds) and if they're not there, go back into gps and highlight the friend and press A to 'go to' again. Repeat until you find them. It doesn't always work first time, so be patient.
5. When you finally find them on the road with you, press left on the d-pad a few times and you will see their name on a player list, possibly with other names. Press down until you highlight their name, and press right on the d-pad a couple of times, following the instructions to 'lock player'. The other person will have to accept the lock request. It's not essential, but it's meant to help keep you together on the server. Now cruise!
Remember though that every time one of you goes into the gps screen, you'll lose contact, and have to do it again. It get's very easy after a couple of times though, so it's no big deal. Another thing, remember that you might still be in private chat, so if anyone else finds you they won't be able to hear you until you go back into game chat. Finally, you cannot join anyone with (N/A) by their name on the gps screen. That could mean they are in a house, race, showroom, shop etc..
I hope that makes sense!