You can purchase a membership directly from the Priority Pass website for $99 (Standard), $329 (Standard Plus), or $469 (Prestige) per year.
The Standard membership allows you and any qualifying guests traveling with you to access any Priority Pass lounge for $35 per person. A Standard Plus membership gives you 10 free visits each year and then $35 per visit, per person. Finally, a Prestige membership gives you unlimited free visits, but you will still need to pay $35 per guest visit.