buggie:

Why it's wrong not to invite them, I do not know.

I wouldn't say it is "wrong" not to invite them, but it would certainly be awkward. Something I haven't seen mentioned is the fact some of the LW's friends are very likely now paired up, meaning her ideal guest list would include some couples while she'd make a point of excluding others. How would you pull that off without alienating people?

If your place is too small to have the whole gang over, you can invite smaller groups or eat out. But telling your friend that since you don't have enough chairs her boyfriend/husband is never going to get an invite is going to be a pretty tough sell.