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" (every step: the send, each signature, a decline, the expiry, a revocation, a delegation, a failed invitation, the reminders and the completion; since 1.4). Each address gets a mail of its own, so nobody on either list sees the others: cc and bcc behave the same.

Watchers: one level each (1.4)

settings.watchers (and signature.watchers in a flow) lists people who follow the request without signing, each with its own level, the same three words as ccNotifyOn:

"watchers": [
  { "email": "requester@acme.example.test", "notifyOn": "everything" },
  { "email": "legal@acme.example.test", "notifyOn": "completion" }
]

"everything" (the default) is every step: sent, each signature, a decline, the expiry, a revocation, a delegation, an invitation that could not be delivered, and the completion. "completion" is the end only, with the signed PDF attached. "none" keeps the watcher on the envelope and tells them nothing. A watcher cannot also be a signer of the same envelope, and an address appears once. Since 1.4 "everything" means the same for cc and bcc: every step, not only sent, reminder and completion. To keep the person who asked for a signature informed, add them as a watcher with "everything".

Full reference: Cc and bcc on an envelope.