Other than getting a cool hat, this whole thing is pointless.
The way the system works is, if you sign up a friend, you get a guide beret (black i think) and your friend gets a sprout beret (white). When you party with your friend, and you both wear your respective beret's, the person with the guide beret has his level capped to match the level of the person with the sprout beret. There are some restrictions, like the level-capping effect doesn't work if the sprout's job is over 50. The purpose of this is to encourage high-levels to party with their friends on jobs they might already have leveled.
Now, that idea has a lot of potential, high level people can help low level friends get through the dunes and such without resorting to powerleveling. The flaw in the whole plan is that when the person wearing the guide beret is under the level-capping effect, they don't get any exp. So, besides just hanging out with your friend and helping them in a party, you get nothing out of it, except maybe some of the items that drop.
If you got exp, and if I knew a friend who wanted to join, I'd try signing up for this. I wouldn't mind donning some low level whm gear to help out a friend, and if someone died, off comes my guide beret for an R3.
If you and your friend stays for long enough, you get to choose between some of the event items that have been handed out in the last year, but for some of them, you could just wait a couple weeks longer and get them anyways when the event rolls around again.
I know SE is starting to get desperate to get more members, but I think they're not going to get as big of a response since you don't get any exp while you help your friends.