Guide of Parties in FFXIV
In a party, you gain some of the experience those other guys get. It Seems that useless since it is divided equally, making it basically training solo with a friend.
Parties can level, too. They get a set amount of EXP for each kill, assuming you are next to someone. At the start it is like 25 or something. After a while it diminishes, but an ADV party is still easy to get. Get your party to level 10, and hit Change to get one!
If these suckers are full and set to contribution, you get bonus EXP, assuming you are next to someone. What you should do is get a friend and take any filler requests for ADV parties. If anymore than two people train in the same place, though, that bonus diminishes. Then you train on whatever gets you decent EXP.
Parties let most anyone in assuming they are online and not harassing people. All they need is the party to be full! At level 20, you start getting awesome party skills that can increase attack, recovery, and drop rate, among other less awesome things, sometimes not worth the points.
When you finish quests with your party members, this is also a kind of teamwork. In the party, you do not need to Buy FFXIV Gil yourself; just share what you have with your party members.