nixos-config/modules/system/accounts/users/custom/jules/default.nix

{ pkgs, config, ... }:
{
  users.users."jules" = {
    initialPassword = "jules";
    isNormalUser = true;
    extraGroups = [
      "audio"
      "video"
      "rtkit"
    ];
    shell = pkgs.fish;
  };
  programs.fish.enable = true;
}