As far as I understand it, an investor can purchase shares in an ITC during an initial public offering or when the company decides to issue additional shares. Both share offerings are relatively infrequent though, so vast majority of share purchases will be from other share owners.
I think the main difference between close-ended and open-ended funds, is that you can always purchase shares directly from the open-ended fund, but in an ITC you will mostly be trading with other shareholders as share issues are rare.
I think when they talk about closed-ended funds being closed to new capital they mean that individuals cannot just go to the company and request more shares. However, I do not think this means that the company cannot issue more shares.