How to address wedding invitations if you're not using an inner and outer envelope
So, if you decide to just use one envelope, how do you address the outside to only the people invited? For families, you can list all the children on the envelope, or if the family is too large, just write “and family" after listing the parents. If you’re not inviting kids, address the invitation to only the parents, and they should understand what that means.
For single people who are allowed to bring a guest, try your best to address the guest by name on the envelope if you know who it is. You write the original invitee’s title and name on the first line, and then write his or her guest’s name and title on the second line, without including the word and between them (that is reserved for married couples).
If you don’t know who someone will bring as a guest, I wouldn’t recommend writing “and guest” on the envelope – it’s too impersonal and cold to address a real person as just someone else’s guest or +1. You can let the person know that they may bring a guest in one of the following ways: 1) include a hand-written note explaining such 2) include a pre-printed card explaining such 3) call or personally tell them. Whichever you choose, ask that he or she write the guest’s full name on the RSVP card, so you can have his or her name for place cards and other correspondence.
Another option is to address the envelope the same way a traditional outer envelope would be addresses (ex. using formal names, and only to parents or only to invitee and not his or her guest), and include a small tag or other decoration around the invitations with the names that would traditionally be included on the inner envelope. So the outer envelope might read “Mr. and Mrs. Jonathan Smith” and the tag would read “John and Susy.” Or, the outer envelope would read “Mr. and Mrs. Benjamin Cole” and the inner tag would read “Ben, Jane, Ella and Aiden.” This is a nice, modern alternative that keeps the spirit of inner and outer envelopes, and adds a little something extra to your invitation suite.