You should definitely ask them. If they require robes but you don't have one, buy some clothes in a neutral colour like white or black, made from a natural cloth such as cotton. If they say you can wear whatever, again, wear what you're comfortable in, but I suggest wearing something clean, special, and again, a natural, not synthetic, fiber.
Not everyone follows this belief, many feel wearing special clothes, like robes, only in circle brings with it an air of magic. I've been to many different circles, the ones where people show up in jeans and a t-shirt they wore all day tend to have a more relaxed feel. Nothing wrong with that if it's something like Yule where the whole event is treated like a party with a circle at some point. If you're having a formal meeting mind you, you might want to set the mood. You don't need robes, but you might want to look into clothes to only be worn in circle.