As discussed in earlier sections, you can interact with people who visit and it can potentially earn you FC. But even if it can't, it's still worth checking up on your guests. There's several things you can do to interact with them, moreso once you reach certain Festival Plaza ranks!
Register as VIP
Whereas the guest list will fluctuate frequently, and visitors you get might not show up again, if you register a guest as a VIP they will remain in your Festival Plaza.
Additionally, when you Battle or Trade via the Internet, there is a guest list (which also lumps in everyone on your friend's list, which can be annoying), and a VIP list.
Register any friends you have on your 3DS who you like to VIP status so that you can easily see them, and register anyone else you want to. You can have a maximum of 50 VIPs.
Sometimes, VIP registration will fail. If so, try disconnecting from the Internet and then talk to them again. It is currently unknown why registration fails and is likely to be game bug related to how the guest list refreshes.
If you see someone wearing an article of clothing you don't yet have (must be same gender as your own trainer, however), they might talk about it and offer to sell it to you for FC.
More expensive clothes will cost astronomical amounts of FC so be careful when making the expenditure. You will need to be at Castle Rank 10 before this option opens up.
Unlocked at Castle Rank 8, this is the biggest reason you'll want to hit it up with your guests and VIPs fairly frequently.
Many facilities are version-exclusive but can be constructed in the game it's not natively present in, but only if you talk to guests, ask them what facility they have to offer, and pay out the FC price to construct (anywhere from 50 FC to over 1500 FC depending on the rank of the facility).
This means that if you play on Moon but would like the Team Orange Dye Shop, for instance, you can find a guest offering that facility, and given that you have FC to spare, you can construct it and start dyeing all your white clothes orange!
This will allow you to fully customize what facilities you want without being limited by the game version you own.