If memory serves, the last time I did this it was considered inappropriate to invite someone without inviting their spouse. My wedding is in my hometown and I'll be inviting a lot of former classmates who I've been in contact with after our reunion and on facebook, phone calls that kind of thing.
in many cases I've never even met their spouses.
EDIT: the main issue I'm wondering about is the awkwardness of admitting that even though I've been communicating with certain people for a while (since the reunion) about 4 years ago, I don't know the names of their spouses! in a few cases I've heard through the grapevine that the marriages are 'having troubles' so I'm not certain if I should invite the spouse. thinking it could make certain people feel bad.
in my case there will probably be room since we are most likely looking at a cake and 'refreshment' reception. it's not going to be formally catered.
mostly it's a case of awkwardness on how to handle not knowing the spouse at all.