Save the Dates

I would address it to "John Smith & Guest" - that way, he knows ahead of time that he will be able to bring a guest.

If you are for sure inviting random plus ones, then you would list it as John Smith and Guest. A significant other, even if they just started dating, is not a plus one and they are invited by name. Be aware that once they are sent, they cannot be uninvited.

I would agree with Lisa. While i don't know if it's required/strongly encouraged per etiquette standards per se, I can tell you that when I would get invited to weddings back when I was single/just starting to date my now spouse/even in a serious relationship with my now spouse, I always appreciated the extra clarity / extra heads up (even if just from a planning/logistical standpoint. Case in point: I received a save-the-date for a wedding a few months into dating my now spouse...the save-the-date was only addressed to me, but I *assumed* (and i know, looking back, I should not have assumed) that since it was an out-of-state wedding, I would be invited with a plus 1. I was so looking forward to attending this out-of-state wedding with my then-boyfriend that, as it got closer, I talked it up to him like "oh, this could be such a fun weekend trip...it'll actually be on our 1-year anniversary weekend....let's start looking at hotels!" I can't tell you how embarrassed I was and how awkward it was to tell my then-boyfriend "Actually, I'm sorry because I assumed earlier, but now I got the invitation, and apparently I'm the only one invited, even though they can easily tell I'm in a relationship (and have been for almost a year by now) by looking at social media." I didn't attend the wedding, and instead I celebrated our 1-year with him. I know I was wrong to assume, but I do wish there would be some rule that "whoever is gonna be on the invitation envelope needs to be on the save-the-date envelope." Well, assuming you know you'll be able to give people +1s with covid guidelines and all.

If you are Inviting John with a girlfriend or person he is in a relationship with, the Save may say John and guest. SO name. When you address the invitation, you need to find out the name of John's SO, and send that person an invitation with their name.
"And guest " not used here.
If not in a relationship, at Save time, just invite him. If you are sure you won't downsize and want to give him a definite plus one, leave a note that says, If you would like to invite a date , let me know her name so I can send her an invitation. And the invitation always goes to her with her name on it. " And guest" is only used when even 2 weeks before invitations go out, neither you nor John knows who is coming. Other than that, you always ask for a name and use it. If there is a possibility you might have to cut down numbers, Send Saves to those in relationships, SO, Saves naming both. Because you invite a couple together, or will cut both together. But you do not have to promise a true plus one unknown guest at this time. If asked by John, you can say you will give plus ones to invite casual dates only if you have the space . But if you put it on the Save, you cannot cut without a personal apology. These days, you may want to just invite truly single with no SO , without commiting yourself to the seat and meal for a date, until invitations go out, by name.

I addressed my envelopes of the single guests just by their name-Example Jane Smith. I didn’t put “& guest” on the envelope.

View Quoted Comment This is what I did as well. I'm saving the plus one information for the invitation

I agree with Michelle and Yasmine. Just send it to the person and add the guest for the invitation. If you put the guest on the STD, you lock yourself in to guests later, and things happen. You may find that closer to the date you can't accommodate random plus 1s for a variety of reasons.