DHK, there was only the twelve apostles of Jesus (the original eleven, and Matthias) as well as Mary, the Mother of Jesus, present in the upper room at Pentecost. 
I would also be correct in the identification of the transition from disciples to apostles. The apostles were appointed just after Jesus' earthly mission, after the resurrection, but before the Ascension, and at varying points between the four Gospels. The most definitive is in the Gospel according to St. Matthew: "Go therefore and make disciples of all the nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit." (28:19) The direction of "go" now establishes the eleven (remaining) disciples as apostles.
In support of the claim for those present at Pentecost, I first reference the transition between chapters one and two of the book of Acts. Chapter one ends with the casting of lots to determine the new apostle, Matthias. Chapter two begins with, "When the day of Pentecost had come...", implying a passing of time between the first chapter and the second chapter. (We also need to remember that when the books were originally written, they were not divided into chapters and verses, so we must approach the writings as a smoothly connected writing, and not choppy and separated.) So, after the passing of time, who was in the room?{ 2:14} answers the question: "But Peter, standing with the eleven..."; we can see by reading the beginning of chapter two that the events flow together in a continuous narrative. Therefore, there is seemingly no transition between the events of Pentecost and the speech of Peter to the crowd.
I must also briefly defend why Mary was in the room with the twelve. We can see throughout the scriptures that Mary was with Jesus every step of His ministry. She was (obviously) with him at his birth; she was with him in the temple; at the wedding feast of Cana; through his ministry; and even at his death and resurrection. On the cross, Jesus gave John to Mary as a caretaker, as a "new son". John took Mary into his home as his own mother. Mary was the perfect disciple; it was her "yes" to God in{ Luke 1} that initiated God's salvific plan on earth. She is the mother of Christ, and she never had any doubt about his Majesty. Therefore, it would only seem to reason that she was with the apostles in the upper room on Pentecost; she would have been there with John, as he was her caretaker, and she would have been there among the friends and disciples of Jesus.
			
			I would also be correct in the identification of the transition from disciples to apostles. The apostles were appointed just after Jesus' earthly mission, after the resurrection, but before the Ascension, and at varying points between the four Gospels. The most definitive is in the Gospel according to St. Matthew: "Go therefore and make disciples of all the nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit." (28:19) The direction of "go" now establishes the eleven (remaining) disciples as apostles.
In support of the claim for those present at Pentecost, I first reference the transition between chapters one and two of the book of Acts. Chapter one ends with the casting of lots to determine the new apostle, Matthias. Chapter two begins with, "When the day of Pentecost had come...", implying a passing of time between the first chapter and the second chapter. (We also need to remember that when the books were originally written, they were not divided into chapters and verses, so we must approach the writings as a smoothly connected writing, and not choppy and separated.) So, after the passing of time, who was in the room?{ 2:14} answers the question: "But Peter, standing with the eleven..."; we can see by reading the beginning of chapter two that the events flow together in a continuous narrative. Therefore, there is seemingly no transition between the events of Pentecost and the speech of Peter to the crowd.
I must also briefly defend why Mary was in the room with the twelve. We can see throughout the scriptures that Mary was with Jesus every step of His ministry. She was (obviously) with him at his birth; she was with him in the temple; at the wedding feast of Cana; through his ministry; and even at his death and resurrection. On the cross, Jesus gave John to Mary as a caretaker, as a "new son". John took Mary into his home as his own mother. Mary was the perfect disciple; it was her "yes" to God in{ Luke 1} that initiated God's salvific plan on earth. She is the mother of Christ, and she never had any doubt about his Majesty. Therefore, it would only seem to reason that she was with the apostles in the upper room on Pentecost; she would have been there with John, as he was her caretaker, and she would have been there among the friends and disciples of Jesus.
