You can put raw ground beef in the slow cooker and it will cook along with the other food.
I sometimes mix ground beef in with some beans or noodles in the slow cooker and it cooks just fine overnight or while I'm away at work.
You can cook the raw ground beef first if you want too and then put it in the slow cooker but putting raw ground beef into the slow cooker is just fine.
The ground beef will cook just fine in the slow cooker as long as you allow enough time for it too cook with the other food which is around 8 hours.