Just reach the rank: Friend with Benefits with the guest characters, which is done by having sex with them after reaching 5 relationship points with them. Then being sure to be in the Eeveeloution form that matches theirs (Gender doesn't matter as long as you're not genderless). Then, there should be a special scene you can view when talking with them just under their character portrait.
Open the game up in your HTML editor of choice and change the line <<set $GrowthQuests to true>> (in version 0.9.2 it seems to be line 2468) to <<set $GrowthQuest to true>> (remove the S at the end of GrowthQuests), save the file, then restart the game from a new game, or a save before that feature is supposed to unlock (talking to Gwen after getting Ditto to transform for the first time). This should fix the issue you're having; it did for me.
edit: itch.io automatically interprets the less than and greater than HTML codes when you put them in a comment; as far as I can tell, you can't escape that function. You'll have to translate the "<" and ">" back to their character codes to find the line, or just use the line number, or just search for the "set $GrowthQuests to true" part.