Jesus had told the apostles to wait in Jerusalem until the Holy Spirit came upon them (Acts 1:4-5). So, after Jesus had ascended into heaven, they obeyed. Walking the kilometre from the Mount of Olives they reached the upper room, inside the city, where they were staying. All the details sound like eye witness narratives, and they are (Luke 1:1-4). Luke, the author of this book, is also keen to document the individuals present. If anybody doubted, they could ask those who were present.