All whose faith had drawn them together held everything in common. That is fellowship as the new Testament understands it, and there is clearly a world of difference between that and mere social activities. The Greek word for fellowship comes from a root meaning common or shared. So fellowship means common participation in something either by giving what you have to the other person or receiving what he or she has. Give and take is the essence of fellowship, and give and take must be the way of fellowship in the common life of the body of Christ.
Christian fellowship is two-dimensional, and it has to be vertical before it can be horizontal.
The person who is not in fellowship with the Father and the Son is no Christian at all, and so cannot share with Christians the realities of their fellowship. Your Father Loves You by James Packer, Harold Shaw Publishers, 1986. Fellowship in the N.T. basically means sharing and self-sacrifice with other believers. This sort of fellowship may even bring persecution.