But if Jesus meant that it was wrong for a Christian to pray in the company of other Christians, many Christians in the New Testament must have disobeyed. For example:
NKJV Act 1:13 And when they had entered, they went up into the upper room where they were staying: Peter, James, John, and Andrew; Philip and Thomas; Bartholomew and Matthew; James the son of Alphaeus and Simon the Zealot; and Judas the son of James.
Act 1:14 These all continued with one accord in prayer and supplication, with the women and Mary the mother of Jesus, and with His brothers.
NKJV Act 16:13 And on the Sabbath day we went out of the city to the riverside, where prayer was customarily made; and we sat down and spoke to the women who met there.
NKJV Act 12:12 So, when he had considered this, he came to the house of Mary, the mother of John whose surname was Mark, where many were gathered together praying.
NKJV Act 16:25 But at midnight Paul and Silas were praying and singing hymns to God, and the prisoners were listening to them.
NKJV Act 1:24 And they prayed and said, "You, O Lord, who know the hearts of all, show which of these two You have chosen
Act 1:25 to take part in this ministry and apostleship from which Judas by transgression fell, that he might go to his own place."
I am sure if I spent longer at it, I could find more examples of Christians praying together. The context of Jesus speaking of praying in private was His teaching against the ostentation that Pharisees engaged in when they prayed in public places for the whole purpose of other people seeing them and praising them for it. He was not forbidding His followers from praying together.