There is no restriction, you can sign up for Priority Pass directly, but their pricing makes no sense for those of us that have access to US issued credit cards. I mean for example the membership plan that best matches the PP access provided by the Amex Plat or CSR they call their "Prestige" level and costs $399 per year and doesn't include 2 free guests. So you're financially better off and get a better product by not buying it directly.