Here's a great free resource on small groups. I put a quote from Mahaney below.
“Many of the most significant changes in my Christian life have taken place in the fellowship of the local church—specifically, in small groups. On many occasions, members of my small group have lovingly (but firmly) confronted my sin and held me accountable as I pursued change. Of course the Holy Spirit is directly responsible for convicting me of these things, but I can only guess where I would be without such faithful friends. God has used them time after time to address sins in my life that I would never have perceived if I were on my own.
It’s been sobering to observe others who have chosen not to participate in a local church or in small groups. They have demonstrated a distinct lack of growth. What’s worse, they haven’t even been aware of their spiritual condition and stagnation. Small groups provide the encouragement, correction, and accountability that keep us from drifting. As important as it is to cultivate a personal relationship with God by practicing the spiritual disciplines, we need others to help us in our pursuit of sanctification” (Why Small Groups, p.6).