Thank you to HappyDenn for all this information.

This command will finds all members of a specified party and returns their names
(or character id or account id depending on the value of "type") into an array
of temporary global variables. There's actually quite a few commands like this
which will fill a special variable with data upon execution and not do anything
else.

Upon executing this,

$@partymembername$[] is a global temporary stringarray which contains all the
names of these party members
(only set when type is 0 or not specified)

$@partymembercid[] is a global temporary number array which contains the
character id of these party members.
(only set when type is 1)

$@partymemberaid[] is a global temporary number array which contains the
account id of these party members.
(only set when type is 2)

$@partymembercount is the number of party members that were found.

The party members will (apparently) be found regardless of whether they are
online or offline. Note that the names come in no particular order.

Be sure to use $@partymembercount to go through this array, and not
'getarraysize', because it is not cleared between runs of 'getpartymember'. If
someone with 7 party members invokes this script, the array would have 7
elements. But if another person calls up the NPC, and he has a party of 5, the
server will not clear the array for you, overwriting the values instead. So in
addition to returning the 5 member names, the 6th and 7th elements from the last
call remain, and you will get 5+2 members, of which the last 2 don't belong to
the new guy's party. $@partymembercount will always contain the correct number,
(5) unlike 'getarraysize()' which will return 7 in this case.