It seems clear to me that koinonia signifies a much closer, more intimate meaning than our modern English word fellowship. As such, we can fellowship with someone (in popular usage, to associate with or jointly participate in an activity with them) without having koinonia, or close, intimate communion. In this way, Jesus could attend weddings and even gatherings of sinners where he was not having “koinonia” with them, but he was, indeed, jointly participating in their activities.
Where does this realization lead? Well, I think it leads us to the fact that we can have fellowship (association and joint participation in activities) without koinonia (intimate communion), but we can’t have koinonia while denying fellowship. To deny fellowship from a brother is to reject koinonia and divide from him, the very thing Paul preaches against so frequently.