You can import students from your Google Classroom group in the three vertical dots > Students > Import students and pictures.
When you select Import students from Google Classroom a new window will open where you must choose the group of Classroom you want to import the students from.
Once the group is selected, the list of the students of the Classroom group and you'll be able to select those you want to import to a new group in Additio.
NOTE: You can deselect Import pictures if you do not want students pictures to be shared in the app.
Once you've finished your students import, you'll be required to link the Google Classroom group (only if it hasn't been linked previously). If you answer NO you'll be able to link the group anytime.