Lymph nodes in groins and liver in pet scan. is this serious?

Possibly: It could be. It depends on what is the cause. A detailed investigation will need to be done.

Probably.: Lymph node that accumulate FDG (positive on pet imaging) can be inflammatory or "reactive". This is pretty rare with lymph nodes near the liver (porta hepatis). If you have a know cancer or lymphoma diagnosis, this would likely represent recurrent disease.