Fellowship springs out of friendship, or it may create friendship. It cements friendship. Fellowship is also a word hard to define. It is friends engaged in a common activity. It is friendship expressed in joint endeavour, friends doing things together. It makes friendship even more enjoyable. It is the happiest human experience. Friendship implies sharing a common possession leading to a common activity on the basis of that sharing.
In Christian fellowship the common possession which Christians share is the Holy Spirit, God himself. As a consequence, the activities they engage in is the work that God is engaged in. They share him, and they share in his work. It is the fellowship of the Holy Spirit in which they partake.
-"The Biblical Concept of Fellowship", pp. 57-58 in D. Broughton Knox, Selected Works Volume II: Church and Ministry