You can create a party either by right clicking your status window and clicking “Create Party” or using the /partycreate command. Leaving the party works the same way or with a command /partyleave. You can invite other players to your party by right clicking their name in a chat window or using the /invite firstname lastname command.
When you’re in a party you’ll see a small status window for each member in the party. By default these windows are visible in both action and GUI mode, but you can click the lock icon to unsticky it which makes it visible only in GUI mode.
The worldmap and minimap shows your party members locations and their names.