Christian fellowship is simply spending time with other Christians in worship, study, evangelizing, doing good deeds, and generally being with others who will encourage your faith. The writer of Hebrews says "23 Let us hold fast the confession of our hope without wavering, for he who promised is faithful. 24 And let us consider how to stir up one another to love and good works, 25 not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near." Hebrews 10:23-25
Satan will do all he can to discourage you and one way he does this is by having nonbelievers exercise negative influence in your life. Believers should not avoid unbelievers, simply do not all them to have spiritual influence in your life. 1 Corinthians 15:33 says "Do not be deceived: 'Bad company ruins good morals.'" Another translation says "Do not be misled: "Bad company corrupts good character'" and still another says "Be not deceived: evil communications corrupt good manners."
Believers should strive to have positive influence on nonbelievers by sharing the love and forgiveness of Christ with them.