Board Thread:The Quarters - Q & A/@comment-180.215.125.0-20150501232228/@comment-24150802-20150502024111

Actually, that's not quite true, although a party with just one person in it is much more common than a truly empty party.

An empty party is one where all of the members have left, even the person who created it. When that happens, no one can ever get back in. Any quest or challenge in that party is lost forever. (Although if you left without meaning to and the challenge had a lot of important information in it, we could actually do something to help you. Go to Help -> Ask a Question if that ever happens. Don't ask about that on the wiki; it's not monitored as much.)

A solo party is a party with one person in it. No one else can see the quest or challenge, unless you invite them to a party.