When you look at a group receiving the baptism, you forgot that the thing they all had in common was unity.
Acts 1:14 (NASB)
[SUP]14 [/SUP]These all with one mind were continually devoting themselves to prayer, along with the women, and Mary the mother of Jesus, and with His brothers.
Acts 2, unity. Cornelius' family, unity.
The baptism can be given to a single person, or a group in unity.
Acts 5:29-32 (NASB)
[SUP]29 [/SUP]But Peter and the apostles answered, "We must obey God rather than men. [SUP]30 [/SUP]"The God of our fathers raised up Jesus, whom you had put to death by hanging Him on a cross. [SUP]31 [/SUP]"He is the one whom God exalted to His right hand as a Prince and a Savior, to grant repentance to Israel, and forgiveness of sins. [SUP]32 [/SUP]"And we are witnesses of these things; and so is the Holy Spirit, whom God has given to those who obey Him."
Now, we all know this isn't the receiving of the Spirit at salvation, because obedience comes after we're saved. This is a gift given to those indeed faithful to the Lord.