If it's the same thing I'm thinking of...
Not only is it for AK Lodge resort guests only, but it is for AK Lodge concierge guests only.
It is only offered a couple of days a week, I think it's around $50 a person and includes a breakfast buffet at Tusker House.
They do offer a Grand Gathering excursion now for parties of 8 or more.
From the Disney website:
Safari Celebration Dinner
Set off on safari in Disney's Animal Kingdom Theme Park. Enjoy a spectacular VIP journey across the rolling savanna. Then dine in style at the Tusker House Restaurant with live entertainment, characters and wild times for all.
Price: Adults and Juniors - $59.99 US, Children (ages 3-9) - $19.99 US (available 90 days prior)