In Flyff,there are several benefits in a party. Some can include people to talk to, more experience, various abilities.If you're the leader of the party, you can also benefit from using party skills.
To properly benefit from being in a party, you should have a partner. Notice the "a" in singular. Having more then two persons in the same area can result in a loss of exp for almost everyone. There are three kinds of partners:
1) The fighting partner. Theoretically the fastest experience gain, although it often doesn't work like that in reality. If both would kill at the same speed,
the experience rate would be much higher of what you'd get by fighting with just a supporter, since you'd gain exp from his kills as well. However, if one of you kills slower,
the other one would suffer and would gain worse experience over time then he would with a supporter. Also, if one of you are an assist, you would probably have to rebuff your partner every fifth minute, taking of time where one (or even both) of you can't fight.
2) The supporter. Most commonly a full support assist, with high-leveled buffs and a good heal, that will make your grind much easier.
If you're not an AoE'er, a standard assist can be quite fine as long as they haven't neglected their buffs.
3) The leecher. Essentially, power leveling someone just to gain more experience. Good if you don't want to spend lots of time searching for a good supporter or so, although it will give you a worse experience rate then either of above alternatives. Also, watch out for if the leecher is a greedy prick that will steal your drops and Flyff Penya.
Thanks to Neophos