Hi guys i just wanna know is there any risk free way to invest bitcoins so they guaranteed grow
It doesn't matter if interest is low but just want to know if there is some place or site in which we invest bitcoins and our bitcoins are secured and we get guaranteed interest(doesn't matter if low)
please help i am newbie in this

First, there is always risk. No Risk=No Reward.
Second, I will assume that you mean getting interest in bitcoin, on bitcoin because there is no controlling the fiat price.
I personally like arbitrage. There is still risk though because whatever you are arbitraging, you still need to hold for a while. There is the risk. A well run lending (service/process/club) would be the safest assuming they are a well run, legitimate group (company, whetever). If those aren't out there yet, they are being built as we speak.