When my organization, ActBeyond, reports statistics about churches and believers, it’s because we have spreadsheets and lists of those entities, and we use the following definitions.
Baptized believers: A person from the people group who has made a profession of faith in Christ (Romans 10) and has received baptism. The baptism has been verified.
Group: a regular meeting to learn to obey the commands of Jesus (not necessarily baptized believers!) and/or baptized followers of Christ. DBS and T4T formats are typical of the group meeting process. These are “seeker-oriented” groups – they do not always become churches, and are not counted among church numbers.
Church: A group of baptized believers who regularly meet and carry out the functions such as described in Acts 2:37-47, have recognized leaders, and have self-identified as a church.
Some movements do not define a church as a church until it starts another ekklesia.
Also, because of the distinctive focus on obedience-based discipleship, combined with studies of Acts 2 and other passages on the responsibilities of the church, some movements will not define a church as a church unless they are feeding the poor, healing the sick, helping the widows, etc.
Finally, we emphasizing discipling whole “households” – pre-existing social units – to faith, rather than grouping strangers. In these situations, leadership is typically already present in the group, and then enhanced by God’s giving of gifts, etc.