DP and me met at uni eight years ago. At uni he was good friends with a couple on his course who are now getting married. I wasn't friends with them, just would have a bit of chit chat when we saw each other but didn't hang out at all.
DP kept in touch with them for two years after uni, but hasn't seen them for years, and there have been a few WhatsApp's exchanges between them.
We were very surprised to get a day invite to their wedding. We are getting married in May, do we need to return the day invite? I want to keep numbers down to make it just family and close friends, and I wouldn't class them as close friends. DP thinks as we are going to theirs it's rude not to return the favour. I think it's an expensive way to think of it!