Christians have always met in small groups – leaders like Jesus and John Wesley built their ministries/movements around them…
Small groups where Christians study, share, pray, and ‘mission’ together come in many sizes and formats. (It’s unlikely that a Pentecostal who ‘prays in the Spirit’ for half an hour will be excited by a Progressive Christians’ group-discussion on prayer where they don’t pray at all, at least in the accepted sense of the word. Evangelicals want more Bible Study; radical Christians more social justice etc. etc.).
Small groups allow for more intimacy, involvement, accountability, than attendance at formal church services. In our John Mark Ministries’ seminars on Small Groups, we opine that, except for unusual factors, 70%+ of regular ‘worshippers’ ought to be in a small group that meets at least fortnightly to participate in intentional personal/group/spiritual growth exercises…
What happens in small groups?
 First, some contemplative silence, centring-down, ‘gathering words’ etc. Maybe, as God’s people have done for millennia, singing in worship/praise together.
 We listen to the Word of God, from a teacher/facilitator/ discussion leader - Bible study, exposition of an idea, book-summary about some aspect of being a Christian in our world…
 We respond by listening to one another – our joys, sorrows, family issues, work-related problems, personal struggles etc. – in a confidential context.
 We pray together: encouraging empathetic intercession for one another: maybe in smaller sub-groups of three or four. Those who find it difficult to pray aloud in a group can mention ideas, or read a prayer etc. And we commit to praying for one another regularly between meetings…
 Because we follow Jesus we involve ourselves in mission: serving the lost/last/least in a ministry-project (in our neighbourhood, overseas via a microfinancing agency etc.). A good book on all this: Alan Hirsch, The Forgotten Ways (seehttp://jmm.aaa.net.au/
More on the theory/practice of small groups: http://jmm.aaa.net.au/
(what does a healthy church – of any size – look like?)