I would just address the envelope to "Mr John Doe" and save the "and guest" for the invitation.

For a party that is informal, I'd write it on the invitation itself, even if you have fully printed invitations. Just add a note that says 'please feel free to bring a guest!". Or put it on a separate piece of note paper and insert it in the envelope.

If you're not comfortable with that, it would, IMO, be perfectly acceptable to put '& Guest' on the envelope.

Sue

We run into this situation at work when we do invitations for certain events. Since the wording on the inside of the invitation is done on the computer, I use two versions. The following is a little formal, but you can tailor it to your own needs: for singles: "May we cordially invite you to...."; and for those who have significant others: "May we cordially invite you and your guest to..."