NOTE: This is a condensed excerpt from my forthcoming (2014) commentary on Philippians.
When Paul speaks of partnership in the gospel, he uses the word koinōnia, which is usually translated fellowship in the NT. He uses the term in a variety of ways to express shared experience among believers with each other and God. His understanding of koinōnia is relentlessly gospel centered and Christ-focused, to such a degree that Paul can assert (via a rhetorical question) that believers and unbelievers cannot experience true fellowship (2 Cor 6:14). Instead believers have been called into fellowship with Christ (1 Cor 1:9), and they experience that fellowship by partaking the Lord’s Supper (1 Cor 10:16) as well as suffering for Christ (Phil 3:10). Believers also experience fellowship with the Holy Spirit (2 Cor 13:13; Phm 1:6).
The Philippians have been participants in the advancement of the gospel. But this more active sense of participation in the gospel must not be severed from the passive sense of mutual experience of the benefits of the gospel in their own lives. It is because the Philippians first shared in the benefits of the gospel with Paul that they were then empowered to participate in the advancement of that same gospel so others might also share in its benefits.
In a day when the term fellowship is loosely applied to any time believers gather together for any purpose, it is essential to regain the biblical understanding of fellowship. What distinguishes true biblical fellowship from simple shared interests and experiences among non-Christians is the gospel centered nature of biblical fellowship. As such it is oriented around encouraging, exhorting, teaching, praying, giving, suffering, etc. with fellow believers in an effort to follow Christ. “The heart of true fellowship is self-sacrificing conformity to a shared vision…Christian fellowship, then, is self-sacrificing conformity to the gospel. There may be overtones of warmth and intimacy, but the heart of the matter is this shared vision of what is of transcendent importance, a vision that calls forth our commitment.”
 D. A. Carson, Basics for Believers: An Exposition of Philippians (Grand Rapids: Baker, 1996), 16.