The actors did get together from what I've heard. As were the 3PO and R2 costumes. Some of the others may have been digitally inserted like Jar Jar and Grievous.
The thing I noticed that I found funny was that you see the OT characters congregate at the end opposite GL. While GL populates his immediate vicinity with PT characters and OT characters that cannot talk back (puppets, costumes, etc.). Think this is any show of animosity between GL and his OT cast?