Is there special pricing for groups? What is the minimum number of people to get the group rate?

Yes they do have group rates for 10 or more honestly that's the best way to go is group

I believe they have group tickets for 10 or more, there is a form you fill in ,depending on tg3 group I guess (school trip, family etc) you would have to call them directly and purchase tickets or at least get a quote

Youth group of 15 or more will receive reduced admission. If it is not Youth group, then 20 or more.