I also generate a groups module arg:

{ lib, ... }@args:

{
  _module.args.groups = let
    tagNames = lib.unique (
      lib.concatLists (
        lib.mapAttrsToList (
          name: host: host.config.deployment.tags
        ) args.nodes
      )
    );
  in lib.genAttrs tagNames (
    tagName: lib.filter (
      host: lib.elem tagName host.config.deployment.tags
    ) (
      lib.attrValues args.nodes
    )
  );
}