John Piper says this:
Your meaning on earth is that God might be known in you and through you. The meaning of small-groups is for people to get together and say how much they need God, and for somebody else to say how much God is adequate for that need. (from Christian Identity and Christian Destiny)
So how does that work? Colin Marshall and Tony Payne explain:
In chapter 5 of Ephesians, when Paul exhorts them to “be filled with the Spirit” rather than with wine, the result will be that they speak to one another “in psalms and hymns and spiritual songs,” as opposed to the kind of speech and singing that tends to follow from too much wine. The work of the indwelling Spirit will lead the Ephesians in spiritual speech to one another, in this case via singing. …
A related point comes out in Colossians 3: “Let the word of Christ dwell in you richly, teaching and admonishing one another in all wisdom, singing psalms and hymns and spiritual songs, with thankfulness in your hearts to God” (Col 3:16). This time it’s the word of Christ that is dwelling in their midst, rather than the Spirit, but the result is the same – which shouldn’t surprise us!
What ensues is godly encouraging speech to one another, in this case teaching and admonishing. Whether the singing is how the teaching takes place, or another result of having the word dwell richly, is hard to say grammatically. It makes very little difference. The point is that all the Colossians are to teach and admonish one another. (from The Trellis and the Vine)
Are you teaching and admonishing fellow believers by helping them see how God is adequate for every messy problem in their life? This can happen to a limited degree on a Sunday morning. It can happen to a much greater degree over dinner or a cup of coffee. But it happens most regularly in a living room where Christians gather for the express purpose of teaching and admonishing each other with the word of Christ.
That’s what community groups are for.