Can people who didn’t vote in the first election, vote in the runoff?
Yes. The November election wasn't really a "primary," in that any voter regardless of political party registration can vote for any candidate regardless of registration. It's really a general election, with the December 6 voting a runoff between the two top candidates. Had Cassidy or Landrieu received 50% plus 1, there would not have been a runoff.
Anyone that is a registered voter they can vote.