Yes, the summit house is definitely open for one and all on race day. Your family can hang out in there. It might be a bit crowded, but they can browse the gift shop and find a table to sit at.
AT makes a good point if you're coming from near sea level, but I would just say make sure your family's well hydrated. Other than that, I disagree about waiting at Memorial Park. I think the summit is an awesome place to watch you (and other racers) finish, and a cool experience overall.