Willow Creek Association is posting a very helpful 3 part series on Assimilation into Small Groups.
Here is an excerpt from the series, taken from the second post-
Broadly speaking, you can group unconnected people like this:
People who are actively looking for a small group. (Relentless)
People who would join a small group if asked. (Ready)
People who would join a small group if asked repeatedly. (Reluctant)
People who would rather be nibbled to death by minnows. (Resistant)
In the first class, folks are actively pursuing community and just need information. They are hungry for community, and will relentlessly pursue it. The Relentless comprise about 10% of your unconnected target.
The second class covers another 50%. An amazingly high number of folks are Ready, and would join a group if someone would just ask them. But no one has, maybe because the church has implemented a strategy designed for the Relentless (too much information, not enough invitation).
The third class presents an interesting but solvable challenge. The Reluctant need time to develop trust, multiple invitations, and a very low barrier to entry (convenient times and locations, lightweight discussions or extended social times, and an easy way to get out.) The Reluctant 20% may need prior relationships with group members or a chance to break the ice before making a commitment to attend.The last 20% of your unconnected are the Resistant. They will never join a group, even though they may say they want to. For example, most men want to be in a small group until they find out they have to join one. They love the vision of community, but the reality is too much for them. The same is true of individuals (or couples) who are given multiple connection options and reject them all. (Every option is too far, too long, too intense, too frequent, or too different.) Love them, pray for them, but don’t waste time designing a system to try to reach them. God can overcome their resistance, but nobody else is going to come close.
I think that Willow Creek is correct is their assessment of where people fall into the four categories they defined. It is important to remember that 60% of the people that come to your church are low hanging fruit - ready to pick. Either with information or an invitation, most people will be ready and eager to get involved in a small group.
Therefore the two keys are clear information (what to expect, where do they meet etc..) and a simple process. If you can nail these down, most people in your church will be ready for a small group. And then you can spend the time to do the rest of the heavy lifting in order to move the other 40% into small groups over time.
Make sure you read the entire series, here are the links to the entire 3 part series