When requesting to take on a quest, post your name along with the name of anyone who is in your party who wants to take on this quest. When you complete the mission, you shall receive your rewards as stated in the quest topic. Experience rewards will divided equally among you and your party and if there is a item reward, all party members shall receive it.
Also, for different parties taking on the same quest, add your name to the topic title so people don't end up posting in the wrong topic and unless it says otherwise, quests are one time only so once you take it, it can't be taken a second time. However, if you want to help someone who has taken a quest you have already done, you can help them but you can not get any of the rewards for the quests, you still get exp for clearing the dungeon though. The maximum amount of people in a party is four.