On all of the party sites that I investigated everyone was painting with acrylic. As a watercolour artist, I don't think the medium is getting a fair shake. If I go ahead with the idea, I intend to host a few Watercolour Painting Parties.
I think an afternoon spent on a covered deck with a drink, some goodies, paper and paint, and some like-minded people would be super fun. And everyone would have a painting to take home too!