Over the course of the relationship, the FI and I have been going to a lot of weddings and getting invitations to them too. And each time, I've been addressed differently.
When we first started dating and his friends started including me on the invite, I was the " & Guest". Totally understandable. I was the added-on guest after they had met me.
Then in the last few months, they've been "Mr. FI FirstName LastName and Ms. Junage LastName". So now they knew us as a couple.
Earlier this summer we got the invitation for FI's cousin's wedding. It was simply "FI & Junage", no last names. I guess I'm in!