In both SS 3.0.0 and 3.0.1, when I click "Add Group" in Security -> Groups, the main frame goes blank, and stays that way no matter what option in the admin is clicked. Admin has to be reloaded at admin/ before anything is functional again.
On OS X and using Firefox 14.0.1. Firebug doesn't show any errors in the console. Same behavior in Safari 6.0.
It seems to be caused by a conflict with the Blog module. When I removed it, I was able to add groups and members.
This is documented in the silverstripe-blog/issues
Installing widgets module fixes the errors.