"When you pray, you shall not be as the hypocrites, for they love to stand and pray in the synagogues and in the corners of the streets, that they may be seen by men. Most assuredly, I tell you, they have received their reward. But you, when you pray, enter into your inner chamber, and having shut your door, pray to your Father who is in secret, and your Father who sees in secret will reward you openly. -- Matthew 6:5,6 -- World English.
Some have claimed that Jesus' words as recorded above would condemn all prayer in public, including what some might call "group prayers," that is, prayers where one person leads a group in prayer, as is the custom in many Christian gatherings. In reality, Jesus was not discussing group prayers at all. The prayers he describes of the "hypocrites" was not a group prayer, but of one who solely prays in public in order to be seen and heard by men. He does not describe these prayers as one person leading a group in prayer, but of individuals who make a performance before men.
The scriptures do indicate that there were what could be called "group prayers" amongst the early Christians. "They prayed, and said..." (Acts 1:24) "They heard it, lifted up their voice to God with one accord, and said..." "And when they prayed." (Acts 4:24,31) "When they had prayed..." (Acts 6:6) "When he had spoken these things, he kneeled down and prayed with them all." (Acts 20:36) "Kneeling down on the beach, we prayed." -- Acts 21:5.
That group prayers were the custom is also indicated in what Paul wrote in 1 Corinthians 14 about such prayers. He speaks of one who speaks a language not understood by the congregation, using himself as an example, saying: "For if I pray in another language, my spirit [breath] prays, but my understanding is unfruitful." (1 Corinthians 14:14) In other words, if he came into a congregation and spoke a prayer in a language not understood by the congregation, it would not bear any fruit in the congregation. since the congregation would not understand what his breath was communicating. And he asks the question: "If you [speaking in a foreign language] bless with the spirit [breath], how will he who fills the place of the unlearned say the 'Amen' at your giving of thanks, seeing he doesn't know what you say? For you most assuredly give thanks well, but the other person is not built up [since the other person did not understand the language in which the prayer was spoken with the breath]?" (1 Corinthians 14:16,17) Thus he says: "I would rather speak five words with my understanding, that I might instruct others also, than ten thousand words in another language." (1 Corinthians 14:19) The fact that Paul speaks of saying "amen" to the prayer of another indicates that group prayers, led by one person in prayer, were indeed the custom in the early church gatherings.
However, the fact of saying "amen" to another's prayer also brings up another thought. It indicates that one's agreement with and participation in the thoughs presented in the prayer is shown by saying "amen" to the prayer. In some groups I have attended, the person who leads in prayer may sometimes say things to which I find that I cannot conscientiously say "amen" to, since, for instance, the words said may promote false doctrine and/or practices that may be idolatrous in nature, etc. In such cases, if possible, I may say my own prayer silently, or simply just refrain from saying "amen" to the prayer. I do believe that one can, at least before God, refrain from participating in a group prayer when his conscience cannot agree with what is being spoken, simply by not adding his "amen" to that prayer.
The one leading in prayer should indeed make sure that he is earnestly praying from the heart to God (James 5:16), and not just to be heard by man, and yet a "group prayer" should be a blessing, edification, to those who hear the prayer, a prayer that they could say 'amen' to. This is part of what Paul discusses in 1 Corinthians 14. Many, however, do pretentiously lead "group prayers" that are very long and involved (Matthew 23:13; Mark 12:40; 20:47), often saying the same things over and over. (Matthew 6:7) Some turn a prayer into a long sermon, resulting in not actually praying to God at all but simply to heard by the audience listening. We cannot, as Jesus, judge the hearts of others, but we can, if we suspect a prayer not be in accord with God's will, refrain from giving our "amen" to that prayer.
Yet our prayer should also be: "Now the God of patience and of comfort grant you to be of the same mind one with another according to Christ Jesus, that with one accord you may with one mouth glorify the God and Father of our Lord Jesus Christ." - Romans 15:5,6.