Tal vez un AppleScript como este pueda ayudar. Tenga en cuenta que no soy muy competente en AppleScript, así que estoy seguro de que esto se puede mejorar, pero nadie ha respondido hasta ahora.
tell application "Mail"
set theSelection to selection
set theMessage to item 1 of theSelection
set theSubject to subject of theMessage
tell application "Address Book"
set theGroup to make new group with properties {name:theSubject}
end tell
set theRecipients to to recipients of item 1 of theMessage
repeat with a from 1 to count theRecipients
set theRecipient to item a of theRecipients
tell application "Address Book"
set theName to name of theRecipient
tell application "Mail" to set theAddress to address of theRecipient
set thePerson to make new person with properties {first name:name of theRecipient}
make new email at end of emails of thePerson with properties {value:theAddress}
add thePerson to theGroup
end tell
end repeat
set theRecipients to cc recipients of item 1 of theMessage
repeat with a from 1 to count theRecipients
set theRecipient to item a of theRecipients
tell application "Address Book"
set theName to name of theRecipient
tell application "Mail" to set theAddress to address of theRecipient
set thePerson to make new person with properties {first name:name of theRecipient}
make new email at end of emails of thePerson with properties {value:theAddress}
add thePerson to theGroup
end tell
end repeat
tell application "Address Book" to save
end tell