First things first, we need a sign! Easy, just download an image like the one to the left and take it to your local office supply store on a flash drive. You can have them print a 4' x 3' black and white image for $4-$5 (varies by location). You can either pin that print directly to your wall or you can go to a hardware store for a big sheet of styrofoam board and adhere the print to that with some spray adhesive like shown here at Sugar Bee Crafts.
There are a thousand different things you can add to this room to give it that witchy appeal. Grab a spare bookshelf or temporarily clear one off for the party. Use it as a display for spell bottles, tarot cards, candles, spell books, bundles of dried herbs, witch runes, mortar & pestal etc. Pinterest and Google are your friends when searching for ideas.
Just be careful not to invoke any angry witches seeking revenge. That could be a disaster for the party! Although a visit from Mr. Northman wouldn't be so bad.