How do I keep someone informed without asking them to sign?

Add their address to cc or bcc in settings — they get a plain notice, never a signing link:

"settings": {
  "documentId": "...", "subject": "...",
  "cc": ["sales@acme.example"],
  "ccNotifyOn": "completion"
}

ccNotifyOn/bccNotifyOn is "none" (default — nobody hears anything), "completion" (one email, once everyone has signed, with the signed PDF attached), or "everything" (also the initial send and every reminder). Each address gets a mail of its own, so nobody on either list sees the others: cc and bcc behave the same.

Full reference: Cc and bcc on an envelope.