My understanding is that clients will be able to use the DLCs of the host, even if they themselves don't have them.
For instance, I've tried enabling Sons of Abraham, and everyone was able:
- to join the game
- to ask gold to the Jews
What isn't clear to me if this boon is exactly like having that DLC, i.e. you could pick Muslim rulers if the host has Sword of Islam, or it's just for the "related" features but does not extend to character selection.