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To invite a close by player to join a party, double left click him/her, choose Invite to Party. A Party can hold a maximum of 5 players.
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After joining your party you'll be able to see the other team mates pictures next to yours.
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Clicking on your party mates picture will bring up a menu where you can choose to Whisper, Add to friend list, Trade, ect.
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Clicking on your own picture you'll bring up a menu that allows you to leave the party.