Fellowship is first the sharing together in a common life with other believers through relationship with God through Jesus Christ. Fellowship is a relationship, rather than an activity. The principle is any activity which follows, should come out of the relationship.
In Acts 2:42 the early church was not merely devoting itself to activities, but to a relationship. It was this relationship which produced an active sharing in other ways. Fellowship means we belong to each other, in a relationship because we share together the common life and enabling grace of Jesus Christ.