Digital currency bitcoin is up six times in value for the year, and smashed through records this month. Friday, it hit a new high of $6,000. And, 49 percent of people surveyed by CNBC think it could go even higher, responding that the currency is on the way to hitting $10,000.

If you feel like you're missing out and wondering whether to invest, Mark Cuban and Tony Robbins have some advice: Only invest money you aren't afraid to lose.

"If you're a true adventurer and you really want to throw the Hail Mary, you might take 10 percent [of your savings] and put it in bitcoin or ethereum," Cuban tells Vanity Fair. "But, if you do that, you've got to pretend you've already lost your money."

In addition to sharp gains, bitcoin is also prone to big drops. It lost about $500 in one day earlier this week after indications of greater U.S. regulation, and briefly lost $2,000 in September after China cracked down on digital currencies. The volatility is one reason why critics are skeptical of the crypto-currency.

So Cuban and Robbins say you need to go into it with the mindset that while you hope you could win big, you know you could lose it all.