When making a party you start off as in experience party. To see this press P on the keypad. On the top right hand coner of the window that popped up, it will have two words. First their is Experience, and the other is Contribution. Since it is not an advance party there would be a dark bubble on the left side of Experience. After your party reaches level 10, it will become and advance party. As the leader of the party you have the choice to leave it on experience (for a lower level player to level fast) or contribution (prefer for higher level players ranging from 25+ that would want to level better).
Experience parties are better for players that are a lower level from the person who is leveling them. Contribution parties are for people that want to earn more experience then without a party (a full party will increase experience gained).