We are all NPC's in someone's game
Had it been a friend who asked I would have invited them in without hesitation, and expected similar response from them if I was the lower level. We do it all the time among the people I hang out with. But allowing a complete stranger to join my team purely to gain from our efforts while offering minimal contribution themselves? That's going to take a little more persuasion than that they would greatly benefit from it, and I was a little perplexed that anyone should even offer this as a reason.
The only conclusion I could come to was that this guy didn't really understand the difference between a regular solo game and an online game. In an ordinary game there is you, and every other character you meet is generated by the computer and run by the game programming to respond to you in specific ways; these are called Non-player characters, or NPC's for short. In a MMOG (Massively Multiplayer Online Game) you have plenty of NPC's, of course, but you also have a lot of characters that are the avatars of other players who, like you, are sitting at home playing the game on their computer. It's really not hard to tell them apart.
I sometimes think there are some people who never quite get that the other characters they team up with are real people. Their attitude is completely selfish and self-centred. They will leave the game in the middle of a big fight without warning, because they have decided to do something else, thereby getting everyone else killed who was relying on them. They will go take a break leaving their character parked in a position where they will continue to receive their share of the experience points being won by the rest of the team even though they have gone off to make coffee or are chatting with friends on IM. And they will consider it a reasonable argument that they should be put in a position where they gain maximum reward for minimum contribution, and are upset when this fails to persuade complete strangers.
You know, I think there are people like that in real life, too.