Post response date invitees...

we asked for our responses ridiculously early (January 1 for our Feb. 21 wedding) thinking that we could send out a second list of invites, but ended up inviting almost everyone that we wanted in the original batch. Plus, we hadn't realized that the response card was dated and therefore couldn't be used again (dumb oversight on our part) Now, I've got a pretty good idea on numbers and it's no where near what we thought we might have...which is definitely a good thing! but now, I have two other couples that I would like to invite. One of which knows they weren't invited in the first place. Is it tacky to invite them now? Do I send a response card even though the date on it has passed? should I include a letter explaining what happened?

I would really like for the to come. Please help!