I could swear I've seen this on here before, but I want to double check. Everyone in a relationship is invited with their SO. But we do have some truly single friends and we didn't give plus ones to single guests (except wedding party, although none of them are bringing dates anyway). If we get more "no" responses than we expected, is it okay to tell single guests they can bring a date if they choose? Or is this still B-listing? Also, what if we can't offer this to all single guests, but only to some? Would that be wrong, or is there some hierarchy (i.e., older guests should have first dibs or something--I don't know) I should follow?
Re: Plus One B-List ?
I agree with the other PPs. It's perfectly fine to offer truly single guests "plus ones" after invites go out.
To me, that's very different from a B-List. A B-List is filling in guests that didn't make the first "cut". And they'll often know or find out they were an afterthought, which is hurtful and rude.
However, offering a plus one (after the fact) for truly single guests is a nicety and courtesy for them. It's thinking of their wants. Which is essentially what etiquette is all about!
Personally, I'd prioritize a plus one for guests who may not know anyone else, then start with immediate family, close friends, move out from there.
I agree with PPs that if you know of anyone on your "invited by name" guest list who's not part of a circle, I'd make sure they know they can bring someone.
Also, it is never a violation of etiquette to be generous.
When I asked the bride a few years ago if I could bring a +1 (I know now that that is a faux pas!) she graciously said that they were having it at a boutique hotel and didn't have enough space but she would let me know later if space opened up. I certainly wasn't offended. I cringe in hindsight about some of the things like that I did as a wedding guest before I knew better.