Bethesda announced that Fallout 76 Wastelanders will bring NPCs to the game on 7 April 2020 which will also be the same day it will release on Steam. There are some quirks between the platforms though, with some things being transferrable and others not.
Fallout 76 Wastelanders will bring two new factions to Appalachia - Raiders and Settlers. The former faction is well-known in the fallout world and is usually not interacted with, other than exchanging the lead, laser or plasma.
However, it appears that the Settlers' plight to the Vault Dwellers will send them to face or even talk with the Raiders. Shooting at each other is definitely not excluded but the trailer definitely showed a player talking with a Raider. That said, it's not that we didn't have such options in previous Fallout games from Bethesda, it's just nice to see the interaction didn't get pulled.
Wastelanders will not be paid DLC apparently as Bethesda made sure to emphasise they are adding the interactable NPCs for free. They did so in a timely fashion as well, barely over a year and a half after release.
Namely, players will not be able to transfer Atoms or Fallout 1st membership balance between the two platforms. This applies to all Atoms, regardless of whether they were farmed or purchased. On the other hand, players will be able to carry over items that were purchased in the Atom Shop.
Furthermore, both Atoms and Fallout 1st membership will be available via Steam as well.