We should be able to throw parties and call someone (aka. text message in long/remote distances (that aren't in your lot's presence).
but we can do that already. IM someone in the game and to throw a party just go to this forum and announce it. Maybe I'm just misunderstanding you ;/
What about outside the forums? I think it would be more reasonable to throw them inside in-game systemacally. Not everybody pick the invation on the forums, especally If it is a short but quick party type.
You will be able to send a message to offline sims in the future. Right now you must call them either on lot (global lot chat) or over private message, while the user is online. No other way.
When you meet someone you like bookmark them. This will allow you to bring his profile up easily. You'll find a "Send a message to this Sim" button on that profile. If you forgot to bookmark them and you have parted company already you zoom out to the map screen and use Search (remember to switch to Sim from Properties) and search for them by name. Be careful to NOT hit enter, though. It breakes the search. Click on the star buttons instead.
Thought we need an option to send the invite to everyone "simultaneously". Other privately, or to public (it will blink or whatever on the city screen popped in).