I'm not aware of a way to invest in Tauchain. If my understanding is correct Tauchain is public infrastructure, is developed entirely on a volunteering basis, may or may not use a native crypto-currency, and if it does it would most likely be distributed by proof-of-work mining. Agoras on the other hand is an application that will be running on top of Tauchain. It is possible to invest in Agoras, but this will only give indirect exposure to Tau's success, and only if Agoras itself turns out to be successful too. It's important to understand this distinction to properly evaluate the risk.