It wasn't work related, it was a student function. It is difficult to get along with people if they all have shared memories that you can't talk about them with. It's a little mean but I don't think it's inexcusable.
While I would have to re-read to know whether it was or wasn't, what was clear was that not only did they generally invite those that worked there, so not inviting someone who's a co-worker when everyone else is going is kind of mean. Especially when you consider that Hotta was a student until 2 years ago (he's been working there 5), and so, not only wasn't invited when he was a student, he now would hardly be as out of touch with the whole "student vibe" as you seem to suggest.
I think the story sadly makes it rather clear that they did exclude him. The fact that they are perfectly capable of inviting him just to get another guy to come kind of makes that obvious.
On a side note- few student functions that I know of are just "student functions". They are normally for a department or faculty, or in some other way specific. That doesn't mean that students from other subjects can't attend or even friends and former students are usually welcome if they happen to be around. At least that's the way it is with most of the universities I'm familiar with. Though it does depend on the event. A barbecue would be a fairly casual thing. Considering this is a convenience store that just hires students, there's little likelihood that they all attend the same department or faculty, and so would automatically be invited to the same student functions.
I find the story interesting in that until now there has been a theme about how people judge others others on superficial qualities. Hotta does it on the train to the other guy (can't remember his name), and turns out to be all wrong about him. He does it as well about the girl only to find out she isn't really that nice. His co-workers have all been judging him, and rather nastily leaving him out of social occasions. The only one who doesn't seem to do so is that guy.
About not including Hotta; who does that?? That is plain wrong- I have seen nothing in the story that would excuse the co-workers giving him that sort of treatment. If you're holding a workplace related social event you invite everyone from the workplace on principle. The only thing that might excuse it would be if a person was really known for getting into arguments and causing trouble with co-workers. Not inviting someone cause you're just not interested in them socially is mean and nasty. And then to only invite them as a means to get someone else to attend is cruel.