A closed IPO refers to IPOs for whom investment dates have passed, they may have been subscribed to and waiting investment allocation and listing on the exchanges.

No, once an IPO has closed you cannot invest in it until the stock is listed on the exchanges, upon listing however you may freely invest in the stock at its listing/market price. However this price may not be the same as what was offered in the IPO. Additionally unlike an IPO listed shares do not have a minimum investment quantity.