Does Acts 2:47 prove eternal security?
The short answer
No. The verse says people "were being saved" — not that they were saved and need not fear hell, nor even that they were saved. "Being saved" describes something ongoing, just as the Church teaches.
What it says
Acts 2:47 says the Lord added to their number those who were being saved.
What it doesn't say
It does not say they were saved and need not fear hell, or even that they were saved.
Ongoing
"Being saved" describes an ongoing action, just as the Church teaches. There is no argument for eternal security in it.