There He tarried with them and baptized

                                      

On the day of Pentecost, Peter preached the gospel to the people.

When he finished preaching, the Bible says:

"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 KJV

Because it says there were “ADDED unto them,” those gathered in the upper room must have already been water baptized in the name of Jesus.

When were they baptized?

After Jesus spoke with Nicodemus about being born again of water and of the Spirit, (John 3:1-21), the first thing the Bible records is:

"After these things came Jesus and his disciples into the land of Judaea; and there he tarried with them, and baptized." - John 3:22 KJV

From this we see that Jesus baptized.

At this time John the Baptist was also baptizing, "For John was not yet cast into prison." - John 3:24 KJV

"Then there arose a question between some of John's disciples and the Jews about purifying. And they came unto John, and said unto him, Rabbi, he that was with thee beyond Jordan, to whom thou barest witness, behold, the same baptizeth, and all men come to him." - John 3:25-26 KJV

Here again the Bible says that Jesus baptized. John the Baptist’s disciples were complaining to John that Jesus was baptizing more people than John the Baptist was.

John responded, "He must increase, but I must decrease." - John 3:30 KJV

Apparently, after Jesus baptized His disciples, His disciples also began baptizing.

"When therefore the Lord knew how the Pharisees had heard that Jesus made and baptized more disciples than John, (though Jesus himself baptized not, but his disciples,) he left Judaea, and departed again into Galilee." - John 4:1-3 KJV

Thus, on the day of Pentecost, all the disciples of Jesus gathered in the upper room that received the Holy Ghost were already baptized in the name of Jesus. There were probably about a hundred and twenty disciples there all together. (Acts 1:15).