How in the Bible were they added to the church?

                                                                                                                                                         

On the Day of Pentecost, when the church began, Peter preached the gospel.

"Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know: Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain: whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it." - Acts 2:22-24

Peter preached to them the gospel of Jesus, of His death, burial, and resurrection.

"Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ." - Acts 2:36

They responded with repentant hearts.

"Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?" - Acts 2:37

Peter told them how to obey the gospel.

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." - Acts 2:38

Peter told them of the promise of the Holy Ghost.

"For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." - Acts 2:39

He urged them to obey the gospel.

"And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation." - Acts 2:40

They then received the gospel and obeyed it. (Mark 16:15-16).

"Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls." - Acts 2:41

This is how they were added to the church.

"And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers. ... praising God, and having favour with all the people. And the Lord added to the church daily such as should be saved." - Acts 2:42, 47