When we repent and accept Jesus as our Savior we come to have a relationship with Him. We become a part of the family of God - the Church. Like we re related to our human families. But, John is talking about something unique. The fellowship John writes about - is not just because we have mutual interests or family relationships or because we come here Sunday after Sunday for services.
Fellowship is much deeper.
Fellowship means experiencing life in Christ. Relationship puts us into the family of God. But, fellowship permits the life of that family to shine through us. Relationship means that all God has is potentially ours. But fellowship means we re actually drawing upon that source.