Go into the main group, and select the sub-group(s), then click on Export to Excel under the "xxx groups selected" header.
The pop-out form will display the selected sub-group(s). Ensure that you have selected "Group Name" as one of the columns to be exported for Include option if you have selected more than a sub-group. That that, the excel will then list the students and the sub-groups they are in. Select the rest of the export attributes accordingly to your requirements. Click on Export button.