This is exactly how you invest in it. Bittrex posted single sell order for 9999 with initial price 1BTC/1CRAIGS, after ICO ends exchange rate will be adjested and you will receive amound of CRAIGs in proportion to how much you invested. YOu can see current exchange rate in the op. Basically to invest you need to buy as much as you desire from that single sell order available. Don't worry - you will not receive 1 Craig per 1 BTC.