[2018] How does the matchmaking work now?

If I host a lobby and I have DLC, can people without DLC join my lobby? (As in them joining through quickmatch)

Provided you're in the lobby awaiting QP players, yep. If the map has begun, only DLC players would be able to join via QP.

Originally posted by Wh♂♂par:

Originally posted by Fatshark Hedge: Provided you're in the lobby awaiting QP players, yep. If the map has begun, only DLC players would be able to join via QP.

NO-DLC users hosting, with a DLC user joining will have DLC maps, too?

Yes.

Scenario. You're the host and you're in the Keep. You do not own the DLC. Someone joins your keep via Quickplay. They DO own the DLC. Now your Quickplay pool of maps includes the DLC maps.