Fellowship speaks of companionship and intimacy. When we put our faith in Jesus Christ we enter into an eternal fellowship with God the Father and God the Son.
24 So you must remain faithful to what you have been taught from the beginning. If you do, you will remain in fellowship with the Son and with the Father. 25 And in this fellowship we enjoy the eternal life he promised us. 26 I am writing these things to warn you about those who want to lead you astray. 27 But you have received the Holy Spirit, and he lives within you, so you don’t need anyone to teach you what is true. For the Spirit teaches you everything you need to know, and what he teaches is true—it is not a lie. So just as he has taught you, remain in fellowship with Christ. (1 John 2:24-27, NLT)
We don't have to wait to go to heaven to enjoy eternal life; eternal life is to know (be intimately acquainted with) Jesus and the Father. This is something we can enjoy and grow in whilst we live this life.
There will, however, be people and things that will try and pull us away from this fellowship by tempting us to live in a way that goes against the teachings of Christ. The Holy Ghost is the third member of the Godhead. He lives in us, teaches us and reminds us how to live according to the ways of God. If we continue to listen to Him and be led by Him then surely we will remain in fellowship with the Father ans Son.