A lot depends on if your guests have been backpacking/tenting/camping before.
I feel like backpacking in inclement weather is an interesting situation for people new to it. I have been out with 2 guys that hiked in rain and got soaked and setup and were stuck in a tent all day and one of the guys loved it, the other hated it. I feel that some people are inclined to like being in situations like that and for others it could ruin their outlook on the outdoors (fair-weather campers ).
+1 on tents for their sleeping arrangements and getting them out even if it is a bit wet. If you are truly concerned, hike in a shorter distance so you are closer to the car and they won't have to hike in the rain too long. With this approach, you can still dayhike if they are feeling frisky or if it ends up not raining at all.
+1 on what you mentioned about bringing a WW tarp. One of the limitations of them being in a tent is if it does end up raining for a long period of time, they are locked in the tent. A group tarp is nice as a hangout spot when out with others (especially those without hammocks).
I've spent much of a day with my hammock hanging buddy just under our separate tarps swinging away while it rained. IMHO it isn't as fun sitting in a stuffy tent all day in the same scenario.
Either way, get them out. IMHO take some precautions so as to not scare them away from the outdoors permanently and all will be fine. Make sure to report back with how it goes.