The IN dimension of church is about fellowship and building community. Jesus prayed that believers would be one. 

Community is a priority for fresh expressions, which seek to make 'communion' part of their everyday lives

This was a prayer for more than unity; it was a prayer for deep fellowship like that between the Father and the Son – may they be one just as you

are in me and I am in you

John 17.21

Believers are to invite each other into their lives. 

The first Christians modelled this:

All who believed were together and had all things in common

Acts 2.44

Church at its best keeps this tradition alive. In Holy Communion, for example, we are reminded that we belong to one another by sharing a common meal. Community is a priority for fresh expressions, which seek to make 'communion' part of their everyday lives.

Community can vary widely - between solid and fluid versions, for example

Some gatherings may worship in very different ways to inherited forms of church, but still be recognisable as congregations: the same group of people meeting in the same place at the same time. Others are more fluid. Community throbs through the week via text and email, and face-to-face meetings occur spontaneously at different times and places. Some fresh expressions are challenging traditional views of what it means to be a Christian community. For a defence of church as a fluid community, see Pete Ward, Liquid Church, Paternoster, 2002.

The Community of Aidan and Hilda, an internationally dispersed community of Christians inspired by the Celtic tradition, maintains contact through a daily rhythm of prayer, a Way of Life, regular personal communication and annual gatherings. The community is providing a spiritual home to people who are both churched and previously unchurched.

Rather than thinking about whether the community takes solid or fluid form, what counts is whether the community is genuine. A weekly gathering does not guarantee community; individuals may remain strangers. Nor does a liquid expression of church; people may be left out. The commitment of individuals to one another is what really matters.

Comment
It's hard to get a realistic impression of any church community without visiting them and getting a feel for what is happening. Genuine community also takes time to develop and effort to sustain. We should be realistic with new communities, giving them time and space to mature.
Beth Keith, The Sheffield Centre

The 'IN' dimension of church may be in tension with the 'OUT'

Members of a group may so enjoy being with each other that their friendships absorb most of their time. Little time and energy is left to make newcomers feel at home. But equally, loving and open communities can be compelling statements of mission in themselves.

'See how these Christians love each other' was not just a passing compliment, it was the single largest dynamic of the early church.

George Lings, Unit 8: Out of sight, out of nothing (Encounters on the Edge #2), 1999, p17

Feltwell Chapel - membersAt Feltwell Chapel, a group of elderly chapel members met weekly for services and fortnightly for Bible study. When a new minister introduced the subject of evangelism, they all revealed a deep dissatisfaction with their own experience of church. Now they are creating community in a way that is attracting other people.