When you say invest in Bitcoins do you mean just buying them and keeping them or is there a way to buy shares in them?
There's no such thing as shares of bitcoin, and anyone selling "shares in bitcoin" is a scammer. 
Buy them and hold them.
Ok so it is possible to buy shares in the growth of Bitcoin if you believe they will go up in price? There is a share called BTC-GROWTH under the FUND tab on BTC-TC, is this a share for Bitcoin if it increases its value in the future? eg if Bitcoin is worth $200 in the future would you make money if you bought shares now?
Read their disclosure:
Quote
BTC Virtual Stock Exchange
BTC-TC is not a real-world stock exchange and does not offer opportunity for direct real-world investment or profit. While we fully expect listed virtual companies to follow through with their virtual business plans, please KEEP IN MIND AT ALL TIMES -- shares purchased on this virtual stock exchange simulation do not entitle you to legal real-world rights to a listed virtual company as you would expect from a real company.
BTC-TC is not a real-world stock exchange and does not offer opportunity for direct real-world investment or profit. While we fully expect listed virtual companies to follow through with their virtual business plans, please KEEP IN MIND AT ALL TIMES -- shares purchased on this virtual stock exchange simulation do not entitle you to legal real-world rights to a listed virtual company as you would expect from a real company.
Buy Bitcoins directly from a reputable exchange and hold them in an offline wallet / paper wallet.