The Church is the Church of the New Covenant. It is the New Covenant community. And Gregg Allison  rightly perceives that apprehension of this reality destroys the popular individualism in much contemporary church culture.
The dilemma: individualism and “contractual ecclesiology.”
In evangelical contexts, the church is often regarded chiefly as a resource for fellowship. For the uniquely individualized personal relationship with Jesus, the church is not only dispensable but perhaps also a hindrance to personal growth. … [A] voluntaristic emphasis emerges, with human decision as the contractual basis for … ecclesial [church] existence. 
Many view the church as a ‘contractual reality,’ i.e., something that comes into existence when fellow Christians just so happen to commit to one another (what is seen as an otherwise optional activity). In other words, the church is the product of Christians deciding to form a community. Thus the church’s existence is thought to be based on fellow ‘contractual’ agreement.
Allison mentions P. T. Forsyth who over half a century ago stated perceptively,
It is a great concern to many and a grief to some to think that what were once Churches among us are ceasing to be such, and are becoming but religious groups loosely organized for family comfort, spiritual culture, or humane action. … [The danger is that churches] would readily subside into a group of mere Christian clubs…. The voluntary nature of the membership tends to reduce such Churches to contractual associations. 
A Church may be joined and used [abused?] for a like reason–for the religious good to be had from religious association rather than out of the love for a common Lord or the sacrificial service of His kingdom. It may be composed of a number of people who have been persuaded that it would be for the good of their souls. But that is not a community, but only a combination. It is not a Church. 
In short, many Christians today view the church as little more (nothing more?) than a means to foster personal spiritual enrichment. … And probably the greater problem is that most Christians fail to see the problem with that sentiment.
The solution: apprehension of the Church’s covenantal nature.
This new covenant relationship between God and Christ-followers is initial, prior to, foundational for, and generative of the covenantal relationship that exists between church members. In a secondary and derivative sense, Christ-followers make a willful choice by faith and in obedience to their Lord to covenant together as a voluntary organization. (Emphasis added) 
What Allison is arguing here is that member-to-member covenant relationship that occurs within the local church is preceded by and based upon God’s establishment of the New Covenant with the Church. The “horizontal” covenant relationship between fellow church members exists first and foremost because of the Church’s (including all of its members) “vertical” New Covenant relationship with God.
Accordingly, becoming a member, joining with others in the voluntary society called the church, does not ultimately constitute the church [better: “…is not what ultimately constitutes the church”]. Rather, it joins that member to an already existing reality, or it defines the constituents of that particular entity that has already been constituted a church…. 
Rather than existing ‘contractually,’ the church is an assumed and necessary reality in the Christian’s life. If one has any participation in Christ, he necessarily has participation with Christ’s body, the Church. And the Church (although universal) does not exist in the abstract, but in spatial-temporal-local congregations.
D.A. Carson puts this in very practical terms.
The Church itself is not made up of natural “friends.” It is made up of natural enemies. … Christians come together, not because they form a natural collocation, but because they have all been saved by Jesus Christ…. They are a band of natural enemies who love one another for Jesus’ sake. 
Because the Christian community’s fundamental reason for existence is a shared relationship to God through Christ, all notions of community which rest on any other commonality must be immediately discarded if we are to speech of a community that can truly be called a church.
In the simplest terms, (1) the Church is not an optional part of the Christian experience. Christian existence is not individual but communal–‘horizontal-covenantal’ by nature of it being ‘vertical-covenantal.’ (2) That community is not a contractual community, but a necessary one. The Christian’s contracting with this community flows out of its necessary existence. And (3), as a corollary of points 1 and 2, participation in the Christian community is not to be driven primarily by a desire for self-benefit (e.g., “spiritual enrichment”), but service, mutual edification, and love for Christ.
 Gregg Allison, Sojourners and Strangers: The Doctrine of the Church (Wheaton, IL: Crossway, 2012). See 128-132.
 Michael Horton, People and Place: A Covenant Ecclesiology (Louisville: Westminster John Know, 2008), 170-171.
 P. T. Forsyth, Church, the Gospel, and Society (London: Independent Press, 1962), 29-30.
 Ibid., 30.
 Allison, Sojourners and Strangers, 128.
 Ibid., 128.
 D.A. Carson, Love in Hard Places (Wheaton, IL: Crossway, 2002), 61.