When we only focus on assimilation of believers or church people we may be missing the point in unchurched culture. Although, I would be the first to agree that too many of our SS classes are closed groups. As leaders, it would seem to me that we would want to develop as many open groups as we can. It wouldn't matter if they met on Sunday mornings at the church or on Mondays at work. What matters most is mission, vision, purpose and direction.
Some small groups are purposed to reach and do an effective job of that.
Others just do ministry. We need to have a handle, as leaders, on what groups are doing what things. We cannot develop new open groups, no matter when or where they meet, if you don't know what they are doing. Once we know what they are doing, we can develop the leaders to continue to accomplish mission, purpose, etc. I'm working with a community-based investment club and gardening club with seeker types, and it's proving to be a great entry point to faith formation and church experience.