It's not easy to do so in real time. Occasionally, an activist investor will reveal a stake in a company in which they're looking to make changes, or an investor will file a Form 4 or related filing as a result of taking a stake of 10% or more in a stock. However, once this information becomes public knowledge, the stock tends to react immediately.
You can also follow along with the top investors' stock purchases and sales every quarter with their 13-F filings, which come out within 45 days after the end of the quarter.
While you won't know the purchases in real time, you'll notice that many of their holdings are consistent from quarter to quarter, so it's easy to invest like investors you want to follow.