Typically if Civil 3D is locking up I always encouarge users to update display drivers, but if you are using Civil 3D 2012 and it locks up while adding or editing multiple Point Groups, Point Group Styles or multiple other Civil object groups or styles in the Prospector. First try hitting the Control (CTRL) key to unlock Civil 3D. This is much better then the alternative of shuting down and re-starting.
To prevent future lock-ups review this hotfix.
Link to Civil 3D 2012 Hotfix