I've often wanted to say, "Good company can 'corrupt' bad habits, too!" But all too often a Christian needs to be sure that their close friendships are with those who will help strengthen their walk with the Lord.
It is true that in the Word of God we find many examples of those who had an impact on their world even though they were alone. Men like Moses, Joshua, Samuel, David, and certainly Jesus, were strong enough to influence the crowd. But, they all (apart from Jesus) had to become strong in the Lord first. This is why Christian fellowship is so important and why establishing strong Christian homes helps prepare both children and their parents to stand strong in a crowd of unbelievers. Don't think you can "corrupt bad habits" easily! Make sure your closest companions are walking with the Lord too. As you grow in the Lord so will your influence grow "in the crowd."