Invitation question

Hi - I have a question about addressing invites. I have a few friends who are engaged but not married yet. Do I address the invite:

Ms. Janet Smith & Guest

OR

Ms. Janet Smith
Mr. Michael Doe

Anyone know? Thanks so much!!

This is how i did it:

on the outer envelope, if i knew both people and they have been together for a while or engaged i wrote:
Ms. Jane Smith & Mr. John Doe

then on the inner:
Jane & John

if you are only doing the outer and you know the people i would say write both names, if not just '& guest' would be ok i think.

I put their fiance's name on my invitation as well, I only put guest if I dont know the person they are dating. I know some people get offended if they are engaged and their fiance is addressed as a guest. I think etiquette says to put

Ms. Jane Smith & Mr. John Doe