Sets which of your back-office employees are working on this loan file.
What you send becomes the complete list: anyone you leave out is taken
off, and anyone new is added. Send an empty list to take everyone off.
Repeating the same id in one request is harmless.

If any single id is not an active back-office employee on your account,
the whole request is refused with 422 and nothing changes at all.
You'll get a 404 if the loan file is not on your account.

Each change is also written onto the loan file, so your staff can see
it where they normally look:

• A note for every employee added or taken off — "File assigned to
  Jane Doe (Processor)", "File removed from …" — which shows up in the
  back office under Notes History. Whether that note is private
  follows your company's setting for system notes.
• The loan file's last-updated time moves when the request adds
  someone. Taking someone off does not move it, which matches what the
  back office does.
• The assignment history records that the change came in through the
  API.
• Every employee added is emailed, the same message the back office
  sends when someone is assigned. Employees taken off are not emailed.
  The mail goes out through the same background process that sends all
  LendingWise mail, so it leaves shortly after your request rather than
  during it.