Hello! I want to add custom checkbox to the add new group form

and then handle with it additional action after new group will be created

But couldnt find any theme or code hook for that! 

You need to create a new app and use the GroupForm extension.

Thanks! So there is no way to add one simple checkbox and write additional action after adding new group without implementing full app by myself?

You can do it via plugin too by extending the form and the save() method but the right way is via app.

And it is not only for new groups; all groups will be affected by this.

Thanks!