This is what I mean by organic. If two characters are in a public place, having a public scene, anyone there can join, or comment on what's going on, whatever. Just like in real life, there are rude and polite ways to insert yourself in a conversation/situation.
Also just like in life (I prefer RP that strives for some level of realism in interpersonal interaction) if you are in a public place and a discussion suddenly turns private, you don't teleport to a broom closet or carry on like you're IN a broom closet, getting angry at those who intrude. You get up, make an excuse, and leave. Or delay the Omg Private moment until later.
Maybe I'm in the minority, but I don't like paging people to ask if it's ok to join them, and I don't like the ooc room discussion that devolves into "oh so sorry! Smileyface smileyface smileyface." and "I'll just ignore this and we'll time stop and pretend we never came in here and you didn't see us ok?" I don't mind a quick ooc 'cool to join in?' just for the sake of being polite, but I think if you are in an IC hangout and you are roleplaying, that's a public scene and anyone can participate.