The co-op mode is basically people joining a single player game and taking over one of the party members (first the non-lead main character, then up to two henchmen), so I'd guess no on support for a persistent server.
The editor supports adding dialog, etc, for the henchmen, so there should eventually be mods designed for 3 or 4 players. The main game focuses on the two main characters, and henchmen don't participate in conversations (though one of the kickstarter stretch goals could add some, at least for specific situations or quests).
