Missional communities come in all shapes and sizes. And this goes for Missional Community leaders too! Different leaders will start and develop a MC in different ways, depending on their strengths and weaknesses. Over the years and in many contexts, my observation is that there are four main, but different, ways Missional Communities develop.
A core team of Christians (around 3 - 8 pioneering leaders) with some others interested & connected around them (anywhere from 0-10 others) start with a missional vision & building community/household in the missional context. This is how the early adopters at a church start to get the vision and live out MC life. Most of these take time to develop in the first & second generations and actually produce fruitful third generation Missional communities. This is because it takes a few stages for the leaders to sharpen their missional vision and for deep relationship to form. It is the leaders that go first and 'give it a try' and, over time, it becomes /normal' and an integrated part of life.
Existing groups (existing average-sized small groups or larger small groups which are almost MC size) start moving towards identifying a missional vision / opportunity / focus and then starting to live towards it/more defined by it. This could mean that a small group becomes more missional itself or that it joins with another small group with a similar missional vision. This is the transition model of how most churches move towards MC transition with the early majority - not moving too fast so not breaking what is in place already.
A small number (2-6 people, always pioneers!) who move into an area (incarnational mission and look for a Person of Peace in the neighborhood) or find a Person of Peace in a network/people group (i.e. a keyholder into a community/existing household) and will actually build community in the person of peace's environment and/or household. This is a model that can work for only approximately 5 to 15 % of the congregation (as it is a high bar for the leader & most communities don't have functional extended households).
There is a need for some expressions of Missional Community for the congregation to experience what is being talked about / suggested and to allow space for the smaller, new Missional Communities to keep their vision strong with those members of the Missional Community. These 'Pastoral' Missional Communities usually have a limited lifespan (6 weeks-6 months) and are used an a pilot experience with which people can experience a Missional Community environment and through it be helped to process whether they should join or start a Missional Community with an authentic missional vision.
Remember Missional Communities are just a vehicle.
There needs to be the engine of discipleship and the passion of missional vision that drives everything. Missional communities are the disciplined first step to help people to start to think & act as missional disciples. They are not an end. The aim is that, through using Missional Communities, they move to discipleship & mission being integrated into their thinking & living and so the structure produces life for your people not programs for your diary.