Depends on whether you're front of house or backstage. Generally, you can't go far wrong with a smart black skirt suit with high heels etc if greeting people out front. If running about backstage, smart black pants with black pumps - Vivier style, for example - will be just fine: even if the teeshirt is horrible, at least people with know that you know how to dress. At a push, brand new black jeans like the ones Helmut Lang used to sell might be acceptable but they have to look shelf-fresh. Pants are OK for front of house too but check with the people you're working for.
Hope this helps.