Have they met your fiancé? Unless in special circumstances (friends who have been living overseas, for example) guests should be acquainted with both of you ahead of “I do”.
No obligations. Just because they invited you to their wedding eight years ago, doesn’t mean you have to invite them to yours. Think of it like this: if you wouldn’t consider inviting them for dinner, it doesn’t make sense to invite them to the big event.
Think future, not past. Will they be in your life in five years’ time? Don’t feel obliged to invite school or university friends if you haven’t been close since. You don’t want to look back on photos and wonder why they were there.
No kidding. Invite kids only if you’re extremely close to them. If any parents ask questions, explain that you want to give everyone the chance to relax and have fun – including them.
Is context key? Apply the “if it wasn’t for” test. “If it wasn’t for work, would I ever see this person?” Save hurt feelings by eliminating entire groups (for example, colleagues or team mates) at a time.
Consult your calendar. When was the last time you saw them? If you live in the same area and it’s been more than a year since you got together they should be a no.
Is that plus one necessary? Unless you have socialised with the couple, there’s no need to invite both halves. (Possible exceptions: a guest who won’t know anyone at the wedding, or the other half has been on the scene for a few years, but the couple lives out of town.)
What do they mean to you? Review any guests on your parents’ list – if you don’t recognise the name they deserve some scrutiny (unless your parents are paying for the entire bash).