You cannot enter Abyssea with a character lower than level 30. And like hg mentioned, it would be quite dangerous. At level 30, you won't be able to hit anything (hell, even at 75 you'll have some trouble at first, barring awesome gear), so you're pretty much going to be restricted to just leeching exp from a party. That's why when you see someone with a low level character in an Abyssea exp party, they're probably buying forbidden keys and opening chests for the alliance. That's exactly how I was able to take ninja from 43 to 90 in ~12 hours that one day.
The other downside to doing that is that it takes quite a while for the exp to build up (due to the aforementioned level penalty). Once the alliance is rolling at full speed, you can get upwards of 600xp per kill, with the mobs going down virtually as fast as they pop up. End result is 100k+ exp/hour, but it takes a couple hours to get to that point. Alternatively, you can see if you can tag along for some dominion ops. The experience reward from doing the dom op significantly boosts your overall rate right from the start, so it makes more sense if you don't have a lot of time to devote to it. Again, the level penalty is still there, but the dom ops reward mitigates it to some extent.