Some people don't feel like chatting during dungeons because they already know what to do and just want to go as quickly as possible. I usually just say "Hello" at the beginning and "Farewell" at the end unless someone else says or asks something.

"/p Text" is the most common way to chat in party, however there's also a couple other ways not many people know about:
  • Typing "/p" then hitting enter will default chat to party. This works with other chat channels as well.
  • You can quickly enter party chat by pressing Alt+P. Other chat channels have hotkeys like this as well, just check your keybindings for what they are.