From a purist theoretical perspective:

You are probably aware, that the motive of each member in a G12 Cell is to disciple at least 12 people during their lifetime. To be more specific, 12 couples in an adult environment. So in essence you could have 24 people in a meeting. Leaders in a pure G12 sense will stop at this point and mentor and disciple these 12 to the best of their ability. Naturally, some people could move out of the circle, like Judas out of Jesus' original 12. He was replaced. In the same way people will move out of your initial 12 and then be replaced by people from the next circle of influence. The idea is for the 12 each to reach 12 making 144 in total. They will at this point, stop reaching out personally, but rather invest in others, causing them to reach out.

If a member of the original 12 (mother cell), reaches out to someone and gets them saved, they will not take the responsibility to disciple this person. They will give this newborn person to one of their 12 (daughter or granddaughter Cell), and they will become part of the 2nd or 3rd generation Cells. A lot of the G12 Churches that we know of, have as a goal, for each member to bring in one person per year. These converts are filtered to a Cell underneath them. The problem with this, is that often the converts do not have relationship with the 2nd or 3rd generational leader. It puts pressure on the leader who brought them in to initiate additional meetings.

From a practical aspect:

Leaders need to constantly be involved in reaching the lost. This keeps them sharp and effective and in touch with the 'world'. What tends to happen practically in pure G12 systems is that the original leaders start additional G12 cells. I.e. they keep their initial 12 and start a new 12 from scratch, reaching out to the lost. This increases their workload tremendously. Or they promote one of their 12 to head up the leadership group of 12 and they initiate a new circle. This last approach has it's own unique sense of complications. The success being however, that instead of only 12 mentored disciples, the person could have effectively mentored 24 people, then 36 etc.

We have seen that the best soul winners are the recent converts. Our focus is on equipping them as quickly as possible while they still have their unsaved contacts to reach out to these people. G12 uses this principle to reach out, i.e train up the converts as quick as you can to get them to reach out win their 12. Whether, you like it or not, at some point the original community in the G12 move more into a mentoring role, equipping others to reach out and evangelise.

A total different way to approach this subject is to look at it from a gifting or an anointing perspective. Each of us are called to reach out to the lost, but let's face it there are some of us that are simply better at evangelising than others. These people who clearly have this gifting should be kept doing what they do best. I.e don't force them into mentoring long term relationships, when clearly their gifting is to evangelise. Have them evangelise, establish Cells, let others take over the mentoring roles and have them move on and continue to reach out. We have seen, that these evangelists get frustrated, with what they see as 'mentoring' and often the mentors, get frustrated with being forced to reach out. I am on dangerous ground, but do you get the point? Perhaps it is because of our focus on discovering everyone's gifting and helping them utilise it to their full potential. There is large turnover in Cell Churches where people are forced into roles that they don't want to assume.

So in short, the only way for a G12 original community to continuously evangelise, is for them to start additional groups of 12 once their original 12 have been found or for them to filter converts into Cells further down the structure. The evangelism paradigm of G12 is on establishing relationship with unbelievers with the idea of a long term mentoring roles.