There are impromtu, "pick-up" events when there's a GM on and that GM has the time and inspiration to do so. They aren't usually announced; you typically have to be in the right place at the right time.
As for the events on the calendar, typically the requirements for those joining are outlined in the entries. There's usually level requirements and such. If there are none, then it's usually open to all. There aren't generally any other qualifications for joining GM-led quests; you don't need to have any particular class, skills, feats, etc.
