Two features, somewhat related.
"party lock" - a marked or selected item, once dropped, can only be picked up by members of current or same party.
"party loot" - only people in the same party can pick up the treasure dropped by monsters slain by other party members.
Some sort of message should be displayed to anyone outside the party who tries to pick up any of these items or loot. (ie, "Only members of party Foo can pick this up, you are not a member of party Foo.")