Just send a news article out to parents telling them how to change their groups.
There's a template in the news section.
1. Go to news and engagement.
2. Create a new article.
3. Don't forget to fill out all the boxes then go down to the text box.
4. In the menu bar click INSERT, scroll down to insert template.
5. Then choose the change groups template and click insert.
You could put your school logo/letter head in first if you want.