A local web page with some JavaScript to convert a list of Moodle groups with students to a searchable list for spreadsheet application. Your data are NOT SENT to an external server, computaiton is done locally on your computer (inside the Web page).
Got to the settings of groups in your course. In the global overview of groups, you should see a list of groups, their users and the number of users within each group (the image was captured on a French Moodle):
Copy paste text into the dedicated area of the MooodleGroupToCsv.html local page. Here for instance, from A1 (left top) to 16 (bottom right on last line).
You can select Flat CSV (default) to get rows containing student, (email, ), group name. Non flat CSV will generate Group name on a single line followed by a list of students in this group (one on each line).
It is usually as easy as open the ".csv" file. You can use this list using vertical search function to associate users to their group.