Bitcoin IS investment already!

Correct, but I think OP wants to multiply his bitcoin while he is waiting, because he could have more if he is holding it, and investing it at a Investment site at the same time, but there is a risk on investing our bitcoin in a HYIP or in a gambling site, because in HYIP, most of them are always running away with all the bitcoin that their investors invested to them, so we have to be careful, or should I say, we don't have to invest our bitcoin if it just going to those HYIP sites our there.
He can never increase the number of coins by only holding as the number of coins will remain the same even in future and if he wants multiply his coins then he have to take risk on investing either in any online investment sites or in trading and I think trading is a better choice.