In a place where having a church building is not allowed by the governing authorities, the church goes “underground.” When this happens, “the church” is associated with a group of believers rather than with a specific place one goes to on the weekend. The Apostles were traveling preachers who started “groups of believers” out of people who were not. Many times they stayed in one place for a long time to make sure the “body of believers” would thrive and multiply (i.e. start more “body of believers” groups).
For more insight see 5 Reasons Established Churches Should Plant Churches.
Adaptive by Design 