I prefer a party to form through roleplay. That is to say, there is roleplay before the character joins the party, not joining the party before "coincidentally" running across the main body and roleplaying joining forces. That's an ideal situation, so I have to give a little slack. Just a little. Watching as characters join the party from across the server until it ballons to a silly size really turns me off, though. I've dropped out of several parties because of that. Beyond five or six characters usually kills a lot of the RP. It tends to devolve into a bash trip. "We have all these people, so lets see what we can kill." Headlong rushing through spawns ensues. Joy.
I normally don't pay much attention to whose portrait has the party leader crown, but I do respect the in-character party leader. Whoever started the expedition is in command, and it is their call who joins the party and who doesn't. Joining a party and then inviting whoever you like is, to me, pretty rude.