TMP is correct that the SG flag is what you're looking for - and specifically the file spawn_cfg_group, IIRC.
'Scaling' can mean many things, though, each with pros and cons:
The LevelUpHenchman() function can be used to upscale monsters to the level of the PC, but that monster's starting HD must be level 1...it won't work on a monster that already has levels. This is good if you're levelling goblins, but not useful if you want to increase the challenge of a grey render. To work with NESS, you'd have to change the script call for the resref such that it pre-identified the encounter as one that should call the LevelUpHenchman() function.
Alternatively, you can approximate scaling by selectively increasing hp, attacks, abilities, bonuses, item quality, spells, and other attributes through a separate function that applies the effects to the creature. There are a few script sets out there that do this. This would also require you to change NESS, as the return value from spawn_cfg_group is a resref string and you'd need to hook your new levelling system into it (e.g. change the call for the resref from a string to a struct, for example, and include the levelling system in the call to spawn_cfg_group to return not just the string, but the levelling info, which would then be applied once NESS spawns the creature).
Lastly, you could get NESS to spawn more of the creatures to increase the 'challenge'.
