Then subscribe to all mods on the MP Modlist, wait for the 'workshop content' downloads to complete. Start steam again and verify game integrity in steam library -> stellaris -> right click name on the game list -> properties.

If there are any version mismatch issues on joining, all players unsub from their current mods and restart steam. How to fix mod related mismatches: If you are subscribed to more than 8 mods: Its usually best to have two modlists per player: One custom for the player to save all the mods they are currently subbed to (as backup) that are not part of the MP game and one list for everyone in the MP game that have the mods of the intended session.

Make sure the Launcher tells you all the same base game version (i.e. No2 cause is people being on a different channel than the host, or their game not being updated from Steam. In that case you should try and restart steam.

No1 cause for version mismatch are usually differing active Mods or Mods that sometimes failed to update in steam. Active DLCs do not matter for Multiplayer joining, since the Host determines the active DLCs of that game (so make sure that the person with most DLCs starts the MP game as host an thus sets the active DLCs for that match - It may be possible that the host can be changed after that and everyone still gets the benefit of any spawned Leviathans etc.)

Stellaris multiplayer different dlc