While exploring Orna, you can form a party of 6 travelers and share experience while in a party together.
In order to start a party, you must first add Allies to your list and invite them through the ally menu. Click the [+] to the right of an ally, and select "Invite to Party". Should they accept, they will have joined your party! You do not need to be within the same area to party up with people, nor to share experience.
While nearby, you and your party share a Party Bonus buff to monster spawn rate and a 10% bonus to all stats. EXP is split evenly between party members based on how many online or recently online party members there are for each player (100%/x, e.g. 2 = 50%, 3 = 33%, and so on) EXP cannot be shared between players more than 25 levels apart.
You may lose the Party Bonus from traveling too far away from each other, but will continue to gain EXP.