Thanks Mark for the link, following is my summary from the link

The Centurion® Lounge
Cost: $50/person
Guest allowed: No limit

Escape Lounges
Cost: free
Guest allowed: 2

Delta Sky Club
Cost: free
Guest allowed: no limit (cost unknown)
Same day Delta or Delta connecting flight boarding pass required

Lufthansa
Cost: free
Guest allowed: no limit (cost unknown)
Same day Lufthansa group flight boarding pass required

Plaza Premium Lounges
Cost: free
Guest allowed: 2 (fee?)

Priority Pass Select
Cost: free
Guest allowed: 2

Additional Global Lounge Collection Partner Lounges
Cost: free
Guest allowed: 0?

Please correct me if I have missed anything. Not much free options, I think only Priority Pass Select is the only one you can use widely and that network sucks in US.