Is there a way to add all of the recipients of an email to a group/folder in contacts from Mail?
Perhaps an AppleScript like this one can help. Note that I'm not really proficient in AppleScript, so I'm sure this can be improved, but no one has answered so far.

  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