In this post, she briefly talks about three scriptural metaphors for the church: temple, living stones, and family. She spends a little more time fleshing out the familial metaphor.
Then, at the end of her post, she includes this very good quote:
When two or three true, born-again believers come together in His name, Jesus is in the midst. Jesus in the midst is church! It is a different experience than Jesus within. We cannot experience Jesus in the midst when we are alone. We can only experience Jesus in the midst when we are in company with others–at least one or two others.
But is it church in the fullest sense of the word? Yes, it is a church in the fullest sense of the word. It is the basic church. You can have more than two or three and it is still a church, but it does not become “more church” because there are more than two or three. It only becomes bigger church.
This is a pretty good explanation. The number of people gathered together does not make a group “more” or “less” church.