What is the biblical basis for group prayer? Mathew 6:6 "But when you pray, go into your room and shut the door and
  pray to your Father who is in secret. And your Father who sees in
  secret will reward you."


I've been curious about this recently, how much prayer is done in public and in groups and how this verse can seem conflicting to that. I know there is some prayer done by churches, the apostles, and giving thanks for meals. But I couldn't find much evidence to say about how that group prayer was done, how private it was, and how much of it was aloud. For example, the following verse is a group is good but that doesn't mean they were even praying out loud?


  Acts 1:14"All these with one accord were devoting themselves to
  prayer, together with the women and Mary the mother of Jesus, and his
  brothers."
When Jesus said to go into your closet to pray, what he was saying is that your prayer is to be between you and your God. Please notice:

All Scripture is quoted from the King James translation.


  Matthew 6:1  Take heed that ye do not your alms before men, to be seen of them: otherwise ye have no reward of your Father which is in heaven. 


The long and the short of it is are you praying to God, or to be heard by man. By saying go into your closet, what Jesus was pointing out is that when you are in your closet the only one who can hear you is God himself.


  Matthew 6:5  And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men. Verily I say unto you, They have their reward.


In Matthew 6:5 is an illustration of what Jesus was saying. Our prayers are a personal conversation between us and our Creator, anyone else who hears it is just eavesdropping. 

That is not meant to say there is anything wrong with praying in public, but when you do make it between you and God, and not for the benefit of others who are listening.