According to the United Airlines website, any military member with military orders can enter any United Club location for free. They ask that you bring military identification, a copy of your orders or leave authorization to show to the United Club representative.
Unfortunately this invitation is not extended to guests.
I used this benefit on my way from Colorado Springs to Washington DC last weekend. I have the American Express Platinum Card but can't enter any United Clubs anymore with it. Thankfully I had my orders and they let me enter free of charge. The United Lounges are nice! They have free snacks, sandwiches, drinks, internet, newspaper, comfy chairs.
If you are curious to learn more, click here to visit the United website