John 6:60 says, "On hearing it, many of his disciples said, “This is a hard teaching. Who can accept it?”" John 6:66 says, "From this time many of his disciples turned back and no longer followed him." The language does not suggest that these "disciples" were guests among the followers of Jesus. There is no distinction amongst those who are there in the crowd with Jesus. The only thing that ultimately distinguishes people within the crowd is their response to the teaching of Jesus. The crowd who was allowed to follow, enjoy the benefits of miraculous meals, and see the miraculous signs of Jesus was a mixture of those who believed that He was God in the flesh and those who hung out for the rewards of being with Jesus but would not commit to His hard teachings. Even Judas was allowed to serve alongside the other disciples despite the fact that Jesus knew the evil that lurked in his heart and the fact that Judas would ultimately hand Jesus over to be crucified.
Michael Frost puts it this way in Exiles.
"In John 6, after Jesus had miraculously fed the five thousand and then walked on water, he believes that it's time to deliver some of his most uncompromising teaching to his followers. Since they had experienced such amazing displays of his power, it seems reasonable to expect that they would be very receptive to his message, but instead John tells us that "many of his disciples said, 'This is a hard teaching. Who can accept it?'" (John 6:60), and "from this time many of his disciples turned back and no longer followed him" (John 6:66). Jesus' "hard teaching" has the effect of separating out true disciples from those who were interested in the miracles but not ready to cross onto the threshold of liminality." (Exiles, page 113)
In Matthew 19, we see Jesus' encounter with the rich young man. Here again the "hard teaching" causes someone to decide not to follow Jesus. Both of these passages assume that the people whom Jesus has invited to journey with Him, and spend time in community with Him, are a mix of true disciples and not-yet disciples.
In The Forgotten Ways, Hirsch speaks of a contemporary example in which a cooperative works together on a mural on a large wall that the city has given them to paint. The initiative comes from a group of believers but the cooperative is mostly made up of non-believers. "The project could take three months to complete, but by the end of it, they have delved deeply into each other's lives, explored many themes that relate to life, God, and spirituality, and have become friends." (p. 226, 227.) Later in the chapter, Hirsch says,
"We find all of these elements in the way Jesus formed his disciples as together they embarked on a journey that took them away from their homes, family, and securities (be they social or religious) and set out on an adventure that involved liminality, risk, action-reflection learning, communitas, and spiritual discovery. On the way their fears of inadequacy and lack of provision faded, only to be replaced by a courageous faith that went on to change the world forever." (p. 241)
I also think we see the principle in the early church, the books of Acts, Romans, Corinthians, Galatians, and Philemon have references to households of faith and churches that met in homes. It appears that the early church was a mix of believers and non-believers. Slaves would be present serving food and caring for children. Some of these slaves were later welcomed into the household of faith as full brothers and sisters. (See Philemon for one example but there are more.) Robert Banks has a great little book that shows (from careful biblical and archaeological study) what it might have been like to attend the church that met in the home of Priscilla and Aquila (Going To Church in the First Century). It shows the interaction of slaves and free people, believers and not-yet followers of Jesus in the church context. It shows how there may have been slaves, nannies, indentured servants, and others present who were given full access to the gospel as they served in the home. Based on all of this biblical precedent, it would appear that the usual route into the Kingdom of God, and to the Church which is known as the Body of Christ, would be a gradual move from being part of the community of believers to being a part of the "courageous faith that went on to change the world forever."