Suggestion: Railroad if a player isn't on all paths. If he is though make the harem official and make a scene with all. If you think one or more characters are not ready to participate sexually make them watch or exclude them. But have the initial conversation together.
If a player isn't on all routes he/she gets railroaded into a scene with one individual LI that they choose.
It's quite likely that the majority of playthroughs have all routes active in some form.
You can also make the outing conversation happen with just the girls that have a active route. But that may make more work because you have to make renders and dialogue for each config.