If you need to grow bitcoin i suggest not to put it on any investment sites now because it's very tempting to them to turn to scam because the price of bitcoin is continue rising. So i were you i will just increase my bitcoins by signature campaigns.
Most of people here, including me earn some money trough signature campaigns. I also think that with participating in signature campaings someone can save some nice amount from payments. I agree that investing in some sites that promise fast profit is bad idea, in the end most people lose their money. But how to grow amount from here is mystery for many of us here, cause we wish to make some serious amount with bitcoins. I chose gambling and for me that is much better idea then trading, or investing in some suspicious site.
I think for growing bitcoins is good to buy it from time to time, but that is for people who have fiat for that.