Each group will be assigned a classic algorithm. Master it! That is devise answers for the following: What problem does it solve? How does it work? What is the worst case complexity? Is there a best case or average case behavior? Use google, the book, Prof. Ninja, each other.
Send one emissary out to each other group, when there they should learn what they can about the other group's algorithm. Then they return to their group and try to explain it. If anyone is unsure of an algorithm go visit that group yourself.