My husband took early retirement at age 56 from a worldwide company. He signed up for his group health insurance to be continued for both of us. I am on Medicare through being disabled for 3 years prior to his retirement. After 1 year 1 month on retirement his
Benefits dept. is saying Medicare is primary for me--Medicare says his group health is primary - Who is Correct and What rule applies to this?

I believe Medicare would be considered your primary since you were signed up for it before your husbands and they've picked up the bulk of your expenses since. I'm not 100% certain though, so you might want to study both policies and see if either states "secondary to Medicare" or something similar.