It's because of the expression "to come along", meaning "to join [people]" Imagine you are having coffee with two friends, and they start talking about going to a live at a bar one of them knows. They speak of it in front of you but are not saying anything that means you're invited, so it's a bit awkward. You might say : "Hey, that sounds great, do you mind if I come along?" But it also means to make progress : "Albert's English is coming along well" Or to hurry up : you're walking down the fields, and your dog is lagging behind you sniffing at things... You will typically turn around and say "come on Ruffus, come along now!" (yes, it's pretty british). So in this extract JKR probably means "to join". Harry (or whoever) feels left out. It could be : Why had ron and hermione got together without inviting him to come as well?

Thanks! Well, I have never concealed my stupidity...

'I can't possibly comprehend' is a strange thing to write. That means 'there is no way that I will ever be able to understand'. That either says that you are too stupid to understand it, or that you don't believe that anyone could ever explain it to you. I don't believe either are true! Along here means 'to accompany them', there is a strong implication that they went somewhere together. I invite someone = they come to me. I invite someone along = they come to meet me and we go somewhere.