Church Metrics is a tool designed to track high level data for the weekend, like Attendance, Salvations, Baptisms, etc. Currently, Church Metrics isn't set up to track small groups (cell groups). Below you'll find some alternative solutions for tracking small groups and church management.
Church Management Software
There are some other great tools out there as well!
Free Options
Although ChurchMetrics was not created to track small groups, there are a few options that you may be able to use to help track small group attendance:
Create a category called "Groups". Create service times for when each group meets.
Create a new category for each small group and input attendance for those
Another option is to use a spreadsheet, in Google Sheets or Excel, and track small group attendance.