Snoop dog, there are lots of vantage points where your freinds can come and watch. Just follow the crowds. But if you're thinking of bringing small children it's worth bearing in mind that there are no 'easy' paths up box hill except by the Zig Zag road.
There are car parks however by the Stepping Stones and at the top there is a National Trust car park and you can walk from there to various points where the race crosses. there's also another car park the runners go through but it's only good for about 10 cars so you're unlikely to get a space.
I wish I'd entered this year, having done the Munro 2 years ago, Best of British eh!
( my punishment that day is swimming down the River Arun with lots of other nutters)