If every person is enrolled in a class:
The group/class is most likely set up as a "Worship Attendance" group. This setting allows anyone in the database, without being on the roster of the group, to be checked present. This option should only be used for actual worship service attendance and not children's classes etc..
To fix this, go to the group/class, click on the blue Edit > Group Information, and uncheck the box for "Adult Worship / Youth Text Attendance" on this page and click "Save."
If a group doesn't appear in the "Add to Group" dropdown:
To add a group to the dropdown list of a check-in view, go to Check-In > View/Settings and click on the blue "Groups" button next to that check-in view.
Select the group/class you want to add and determine if you want security labels, name tags, or no labels for this group and click "Save."