Returns one activity recipient record by its ID, under activityRecipient. The linked activity entry comes beside it, under activities. Returns 404 if no record has the id. Returns 403 when the linked activity entry cannot be read. That happens when the activity entry no longer exists, or it is filtered out for this caller: a deal activity when the caller has no deal access, a mailbox email in another user's private mailbox, an entry for a deal the caller does not own (without permission to see all deals), or a list subscription that was removed.