1. They can do it themselves. Trillions of dollars are invested this way.
(Of course, the only problem here is that most people have no idea how to analyze and choose individual stocks. Well, not really the only problem. Most investors have no idea how to construct a stock portfolio, most have no idea when to buy and sell, and most have no idea how much to invest in the first place.)
2. They can give it to professionals to invest. Trillions of dollars are invested this way.
(Unfortunately most professionals actually underperform the market averages over time. In fact,it may be even harder to pick good professional managers than it is to pick good individual stocks.)
3. They can invest in traditional index funds. Trillions of dollars are also invested this way.(The problem is that investing this way is seriously flawed--and almost a guarantee of subpar investment returns over time.)
4. They can read The Big Secret for the Small Investor and do something else. Not much is invested this way. Yet...