So, in WoW, right, there are these 'party' things, right, and like, if people go by and you tell them to invite you then you'll join their party and they'll help you with stuff, right.

Not exactly.

I'm a big fan of teamwork in games. It's one of my main reasons for playing MMOs. Mostly I like to be in a team that I know works: one composed of people I know and trust. I'm happy to take the plunge and start random groups with people who ask nicely, with the extent to which I'll go out of my way depending on just how nicely.

I don't mind if it's not unbroken English provided there's a tiny bit of thought there; a 'pls' or a 'plx' may not be as good as 'please', but it's orders of magnitude better than taking for granted that I will start a group with you. That goes double when I'm already in a group (assuming I even have the authority to make the decision).

'inv' is not a polite request for an invitation. Attempts to clarify what was meant with another letter or two each time don't constitute additional politeness. Presumably he'd given up on the invitation by the time he decided to tell me I sucked. I would have had much more time for him had my Better Half not been in the group with me; she's fairly new to the game and gets confused enough when I'm running around getting quests done for us. A random stranger wasn't going to help that.

And indeed, didn't. Later, the prodigal random bumped into us again, this time asking for help with a particular fight. Quite why he couldn't manage it already I don't know, but anyone can get to quite high level in WoW before they need even the slightest clue of how to play.

I invited him. The more I think about it the more I wonder why, but he asked for help so I gave it to him: I made sure he was in the group then ran and killed the NPC he was after. I asked in party chat whether he'd got it (the head, or whatever other credit that quest gives); he responded that he was just going to get the quests we were on (actually the quests we were more or less finished).

Which brings me to another crucial piece of party etiquette: don't outstay your welcome. Existing parties that you bump into may be guild-mates (you can check that one easily enough), real-life friends, family, or some other group that you don't belong to, and unless they're genuinely happy to have you along you should take care not to be there longer than everyone has agreed to.
If you are invited to an established party for a particular reason, thank them and leave as soon as you're done. To do otherwise is kind of like knocking at a random house and asking to use the toilet, then hanging around afterward as though you were a known guest.

If you group up with someone who was on their own then it's less of an issue, since (in WoW at least) they can leave the group and it will be disbanded automatically. If there's more than one of them they can kick you out of the group, but I for one don't like kicking people and it irritates me all the more to be forced to do so (to the extent that I'll normally just let them stay in the group and carry on as if they weren't there: I'm sitting next to the Better Half so it's not like I need to type on the party chat channel).

I feel mean now.
To be clear, I don't mind people being no good at the game: there are plenty of folks who mock requests for help on solo missions (or even some group ones) on the basis that they can be done by a single player, but I'm only too happy to help someone who looks like they'd appreciate it. Some solo quests are actually really hard to do on your own, just as some group quests can be solo'd; often circumstances can make a quest very hard for some classes compared to others.
For serious group work people ought to be up to a certain standard and you sometimes see people taking the mick assuming they deserve a place in a party when they clearly can't pull their weight, but I tend to be soft even on them. In several years of MMOs I've never left group content before we'd achieved our goals or the rest of the group had given up (and that's cost me many painful ours in bad groups who clearly won't going to manage what they planned).

I just don't like being taken for granted.