that could cause some really bad stuff to develop with the dog, just IMO of course. They thrive on being social, they will desperately want to see visiting family members, by crating them away you could produce a crazy dysfunctional dog. Any dobie is going to know the difference between friends & family and a person with bad intent, you don't need any special training for that.
Even if a friend you trust is thinking bad thoughts the dogs will know, I've seen it happen many times. You want to allow them to love your family members which is a normal pack instinct.