Party lingo
I think there was a page on the wiki that covers things like this, but I don’t remember for sure.
In the meantime…
2/5 – when you see that, it’s probably the party size, as in 2 people, space for 3.
self invite – when you right click on someone’s name, if they’re in a party, it should give Join Party as an option, and then they accept. People join parties during farming events and the like to make it easier to tag things for loot.
lfg – looking for group. These people are not in a party, and want to join one for better tagging and loot.
http://wiki.guildwars2.com/wiki/Abbreviations has some more that might help, but it still might not cover everything that you’d run into, and I’m probably too used to the language ingame that it doesn’t come to mind right now.