Ecclesiastes 4:9-12 points out the obvious fact that two are better than one. When we pray with others, we share the spiritual load of what’s troubling us, bearing each other’s burdens like Paul tells us to in Galatians 6:2.
We’re also reminded in Romans 12:15 to rejoice with those who rejoice and weep with those who weep. We can thank God with our friends for the good things He’s done in our lives. We can also come before Him with sadness when things aren’t going well and we need His help.
Finally, in Matthew 18:19-20, Jesus tells us that when two or more people pray together in His name, He is with them. There is power in prayer and there is power when we pray according to God’s will together.