Christians are usually accused of "judging" people whenever they speak out against sinful activity. This is not what Matthew 7:1 is really talking about. When it states in Matthew 7:1 "Do not judge, or you too will be judged." it is talking about judging hypocritically. A lot of people will speak out against someone sining yet, are doing the same thing they are doing or even worse! This is what Jesus does NOT want us to do.
However, He DOES want us to lovingly correct wrongdoing or sinful behavior. In Matthew 18:15-17 it states, "“If your brother sins against you, go and show him his fault, just between the two of you. If he listens to you, you have won your brother over. But if he will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses". This is speaking the truth in love (Ephesians 4:5) and in doing so is helping the person out and spreading the Gospel.
James 5:20 "remember this: Whoever turns a sinner from the error of his way will save him from death and cover over a multitude of sins."
2 Timothy 4:2 "Preach the Word; be prepared in season and out of season; correct, rebuke and encourage—with great patience and careful instruction."