Whups. I dumped my add-contacts code in the wrong if statement, so they weren't created unless the group already existed.
Fixed.
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tell application "Contacts"
set contactsToAdd to people whose last name starts with "a"
if length of contactsToAdd > 0 then
if (group "New_Sort" exists) = false then
set newGroup to make new group with properties {name:"New_Sort"}
tell newGroup
save
set selected of it to true
end tell
else
tell group "New_Sort"
if selected of it = false then set selected of it to true
if (count of its people) > 0 then
delete every person of it
save
end if
end tell
end if
repeat with i in contactsToAdd
add i to group "New_Sort"
end repeat
save group "New_Sort"
end if
end tell
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