Why does it appear that everyone in the database is enrolled in a group/class in Churchteams?

Worship Groups allow for everyone in the database as a potential attendee, but should only be used for Worship attendance

The group/class is most likely set up as a "Worship Attendance" group.  This setting allows anyone in the database to potentially be checked as present without being on the roster.  This option should only be used for actual worship service attendance and not small groups or children's ministry classes. 

To fix this, go to the group/class, click on the blue Edit button, then Group Information.  Uncheck the worship attendance group option on this page.