Basic commonalities:
* They're all at the same party. This seems obvious, but it's the most fundamental thing they share.
* They're likely all social. Parties are social gatherings, so people who attend generally enjoy being around others.
* They likely share some common interests. Parties are often themed or organized around a specific activity, so attendees may have some interests in common.
More abstract commonalities:
* They're all looking for something. Whether it's connection, fun, distraction, or something else, everyone at a party is seeking a specific experience.
* They're all part of a temporary social structure. For the duration of the party, they are all part of a unique group with its own dynamics.
Humorous/Unexpected:
* They're all probably wearing clothes. Unless it's a very unusual party!
* They all have to go home eventually. Parties can't last forever.
Ultimately, the most specific commonality depends on the type of party. A book club party will have different shared interests than a sporting event party, for example.