I am trying to create a save the date invitation. The bride wants the invitations to have rounded edges of about .5 inches. Is this something I can do to the art board itself, or do I just create a rounded edge rectangle, put my art on that, and specify to the printers how to cut the art?
You can't round an artboard.
You could use a rounded rectangle, but you shouldn't.
Corner rounding on the printed piece is handled after the piece is printed and when it is being folded and cut.
Design with square edges and don't do anything to indicate rounded corners. Then tell the printer you want the corners rounded when they print it. (note, this may increase printing costs.)
Scott is correct that AI artboards cannot be rounded. If you are creating for web, I would suggest rounding corners in CSS, so design as you would in print with square corners. If you are creating for devices… I don't know. BUT, if you must round corners in AI, you do that to a rounded corner rectangle (or use live corners as in recent versions). From there, you create a PNG or SVG with transparency.
If creating a PNG, you can “Save for web” and uncheck the “Clip to artboard” check box (near the sizing options). If creating an SVG, it is important to use an artboard at the exact width & height (in pixels) as your graphic.