In Acts, the use of the word “witnesses” relates to the Apostles themselves, who were “witnesses of his resurrection.” It doesn’t seem to be used of the normal believer throughout Acts.
Acts 1:8 was a commission and a promise given to the Apostles only, which was fulfilled in their missionary journeys (Mk 16:15; Col 1:23). See it in that light:
“But you will receive power when the Holy Spirit has come upon you, and you will be my witnesses in Jerusalem and in all Judea and Samaria, and to the end of the earth.”
At Pentecost, the rest of us get our marching orders however; there’s more to do as seen in the Joel quote by Peter. In that quote out of Joel below, “prophesy,” in my view, is in its most general use of speaking out a message from God. In this case, it is the gospel. He includes prophesy again when not found in the Joel quote in the words “and they shall prophesy,” giving it emphasis. Peter appears to be saying that in the period between this experience at Pentecost and the Day of the Lord is a harvest time when the gospel explodes across the entire world! And “everyone” (Jew or Gentile) who calls on the name of the Lord shall be saved.”
Read it again carefully here:
“But Peter, standing with the eleven, lifted up his voice and addressed them:
“Men of Judea and all who dwell in Jerusalem, let this be known to you, and give ear to my words. For these people are not drunk, as you suppose, since it is only the third hour of the day. But this is what was uttered through the prophet Joel:
“‘And in the last days it shall be, God declares, that I will pour out my Spirit on all flesh, and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams; even on my male servants and female servants in those days I will pour out my Spirit, and they shall prophesy.
“And I will show wonders in the heavens above and signs on the earth below, blood, and fire, and vapor of smoke; the sun shall be turned to darkness and the moon to blood, before the day of the Lord comes, the great and magnificent day.
“And it shall come to pass that everyone who calls upon the name of the Lord shall be saved.’” (Acts 2:14-21)
Acts 1:22 “ . . . beginning from the baptism of John until the day when he was taken up from us—one of these men must become with us a witness to his resurrection.”
Acts 2:32 “This Jesus God raised up, and of that we all are witnesses” (concerning the apostles who saw his resurrection).
Acts 3:15 “. . . and you killed the Author of life, whom God raised from the dead. To this we are witnesses.”
Acts 5:32 “And we are witnesses of these things . . .”
Acts 10:39 “And we are witnesses of all that he did both in the country of the Jews and in Jerusalem. They put him to death by hanging him on a tree”
Acts 10:41 “ . . . not to all the people but to us who head been chosen by God as witnesses, who ate and drank with him after he rose from the dead.”
Acts 13:31 “ . . . and for many days he appeared to those who had come up with him from Galilee to Jerusalem, who are now his witnesses to the people.”
We should be grateful to God for his amazing plan, which is to show himself as one alive from the dead to the Apostles and to appoint them for this special task, which is foundational for our faith. We continue the work of spreading the gospel of Jesus’ death and resurrection even though we are not the firsthand witnesses.