Invitation wording

What do you do when you know the person is married, but don't know what their spouse's name is? For example, I have several people from work that I am inviting. I know some of them are married, but I don't know what their husbands and wives names are. Is it OK to address the outside only to them, and put 'Mary and Guest' or 'John and Guest' even though I know their guest will be their husband or wife? Or should I try to find out what their names are?

I did this:

For guys, I addressed Mr. and Mrs. Smith.

For women, I asked if they had changed their maiden name, and if they did then it was the same (Mr. and Mrs.), but if they kept the maiden name, then I asked for the full name of their husband.

Good thinking! Thanks ladies!

Yes, that's what I thought too. But I like the 'Mr. and Mrs.' idea, which I also think would be appropriate because they are my bosses and co-workers. Everyone else's names I know (I hope!)

Thanks again!