My personal opinion: Small groups seem like a good idea, as long as they are guided by the proper Church authority (priest), remain in union with the parish, and the teachings of the Catholic Church. I’m sure there are billions of them out there in Catholic Churches as I’ve seen them in mine. In fact, it seems to me that historically, many Orders started out as small groups. Jesus himself had a small group of 12…and these apostles (except Judas) became the shepherds of the Church and have carried it on through the bishops, even to your priests and deacons. So like I said, small groups are good, but they must be obedient to His ordained authorities and in complete union with the Church. Plus, small groups seem in line with the principle of subsidiarity (correct me if I’m wrong).
Now, as others have pointed out, I’ve seen and heard of many Protestant small (cell) groups who without the proper understanding of authority - lead and teach their own interpretations. I’ve met some, who discouraged by their small disagreements with mainstream churches, only meet in small groups of people they can agree with, instead of attending church with the rest of the community. The only Protestant churches I’ve seen that succeed in this are ones with incredible authority structures that organize all the small group leaders and train them before starting any group.
For instance, I was once a missionary in China. There, many Christians (Catholic and Protestant) meet in houses. As far as Protestant house churches go, they are grouped into “movements” of thousands usually meticulously run by gifted “elders” who take on authority much like Catholic Bishops. These Christians are forced to do small groups, because they have no other options in the face of persecution.
As far as using a cell group model in Catholic parishes for greater social benefits and evangelization, that seems like something the priest would have to want and support. As far as for evangelization or chatechesis goes, I don’t see why a priest would object, seeing that this would help to energize his parish. Solid discipleship does grow the Church in many ways, including the fact that priests (increase their numbers Lord) are born from ordinary families, and I can’t think of a better cell group structure than the family!