Pastor's Perspective:
If you want them to be evangelistic, you need to build it into them from the start because the tendency will be towards "the holy huddle."
When we transitioned from Sunday School model to cell model, the groups were vibrant at first, but a few things happened (or didn't happen).
1. They didn't reach out effectively. They greatly strengthened the bonds of already existing relationships, but there was very little new blood.
2. The groups that were "more attractive" grew via "transfer growth," much like what happens in church-dom. A couple of groups folded, while a couple of groups grew and didn't want to multiply. Our 6 healthy groups have in essence become 2 really BIG pseudo-healthy groups and a couple on life-support.
If possible, set in place the process of multiplication beforehand or evangelism will be stunted (I tried unsuccessfully). Convince each group that their goal is to grow and multiply. That means from the start, they're already determining the 'who, how, etc' of the next group.
Sometimes the only hope is to do a relaunch of the groups. Set up your target audiences, identify leaders (probably different ones from before) and set them out to recruit their groups. We did this once with great success, but alas, back to the holy huddle
I knew of one pastor who's rule of thumb was: any group that didn't multiply in 6-9 months got the plug pulled. Not sure if he truly enforced it or just threatened with it.
*Since your people like the "church facility-event" style, you might consider using the small group launch as part of an evangelistic thrust Alpha Course-style. You have a dinner (at the church perhaps) where people invite friends, family, etc and they sit at tables, have dinner, then listen to a "talk." They are invited to return and subsequent weeks and have dinner together at the same tables and build relationships while being evangelized/discipled. Each table is a "small group" with a leader (pre-determined by you, of course). When the series is over (10-13 weeks) you can encourage them to continue as a small group, meeting wherever. I like the concept and our church is considering it as a strategy.*
Long-winded, sorry.