Hi Tania, and welcome.
[y]
Originally posted by taniamunk
Can one purchase a membership to the Clubhouse? Can one purchase a yearly gold card?
Yes - it costs between $10,000 and $30,000 per year, and comes with 4 free Upper Class flights...

Originally posted by taniamunk
If not what does it take other than flying UC or the Gold Card to get an invite to the lounges?
We fly to London 3-4 times a year on economy (bargain fares)
Seriously, though, it's not an option to buy into the CH. There are other ways, when flying PE, however (although these are predominantly for London).
1) Is through FlyingCo. vouchers, if you work for a company that is enrolled into it.
2) Is what several other members here use - the Amex Centurion card link with VS (if you have their card, you can get in when flying with VS in any class)
3) Is to buy entry with Virgin Holidays (This will be your most likely option, at a guess). The good news is, it's possible. The bad news is it's 65 quid per person, per visit (from memory) if you wish to use it. That is only valid, however, for the London Clubhouses.
I have a feeling you may be out of luck with the US stations... [:#]