There will be an opportunity for drumming and dancing, and those who have animal masks are welcome to wear them. If you have drums or rhythm instruments, bring them as well. Runes can be drawn over the cauldron and blessings given as well. Everyone will have a chance to stir the concoction as we all chant the spell. When the energy peaks and the stew is done, we'll share it, along with whatever other food and drink people bring along.
Finger food is probably best.
Wear clothes you can dance in. The deities most relevant to this celebration are Odin, who leads spirits through the skies, and the Vanir, especially Freyja and her older, witchier aspect, Heide. Trance possession is not the goal of this ritual, but rather the kind of contact high you get at a really good party. It is possible, however, that some people will find themselves in some level of altered consciousness. If you want to avoid this, don't wear a mask.