I'm actually in a similar situation at the moment where I had an international wedding and am now having a US Reception months later for those who couldn't make it. Our restaurant required a minimum of 120 guests to book the whole place (otherwise customers could come in and out). However due to a number of circumstances, we overestimated and now have about 100 confirmed guests.
I'm in the awkward situation now of wondering if 2 weeks before the wedding I can just ask some more distant friends if they'd like to come (which means they'd know they are just there to fill a seat of free food and drinks) or skip the awkwardness of that invite and just eat the cost of the 20 empty "seats".
I wouldn't expect gifts from these "extra guests" but I still feel some people, no matter how distant our relationships, might be offended if I just said: "Hey we were a bit off with our numbers and were a few under the minimum our venue requires, so I was wondering if you wanted to come and enjoy some free food and drinks and celebrate our wedding in 2 weeks, no gift expected."
Guess this thread is a good place to ask peoples' opinions, since it's a related topic.