Pentecostals cover a wide range of beliefs, but as a rule they accept the Bible as authoritative.
If we do a bit of deduction, I think we can get a pretty good idea of the answer to this question from the Bible.
First we establish that those who are written in the book of life are those who will be "saved" "go to heaven" "elect" however you want to phrase it.
See Daniel 12:1
“At that time Michael shall stand up,
The great prince who stands watch over the sons of your people;
And there shall be a time of trouble,
Such as never was since there was a nation,
Even to that time.
And at that time your people shall be delivered,
Every one who is found written in the book.
We can make the small jump to the idea that this book is not exclusively about the people in the end days, but includes people going back even to the 12 apostles,
Luke 10:20 (NKJV)
Nevertheless do not rejoice in this, that the spirits are subject to you, but rather rejoice because your names are written in heaven.”
Paul establishes even more that those who share in the Gospel are written in the book
(Phil 4:3) (NKJV)
And I urge you also, true companion, help these women who labored with me in the gospel, with Clement also, and the rest of my fellow workers, whose names are in the Book of Life.
Now having established that being written in the book of life is equivalent to being marked out for salvation, we can see what the Bible says about salvation, to see when and how these people were added to the Book of Life.
Paul states this most plainly in Ephesians 1:3-6 (NKJV)
3 Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, 4 just as He chose us in Him before the foundation of the world, that we should be holy and without blame before Him in love, 5 having predestined us to adoption as sons by Jesus Christ to Himself, according to the good pleasure of His will, 6 to the praise of the glory of His grace, by which He [a]made us accepted in the Beloved.
2 Thess 2:13-14 (NKJV)
13 But we are bound to give thanks to God always for you, brethren beloved by the Lord, because God from the beginning chose you for salvation through sanctification by the Spirit and belief in the truth, 14 to which He called you by our gospel, for the obtaining of the glory of our Lord Jesus Christ.
So the Bible tends to use language like "From the beginning" or "Before the Foundation of the earth".