What's the etiquette for who gets to invite a plus-one and who doesn't?

Partners are normally invited if you socialise with them as a couple. For example - If you are inviting someone from work but have not socialised with them outside of work with their partner, you are ok to invite just them alone.

The best way for you to make sure it is clear as to who is invited to your wedding is to have their names printed on the invite. Yes, you can print their names on the envelope, but we have found the point of who you are inviting gets lost and you are left with an awkward situation of saying that can't bring their partner after their have RSVP.  

If you are providing a post back RSVP card, you can also have the guest names pre-printed on there to make extra clear who is invited.

I would also suggest putting it on the invite, the actual guest’s name of that partner as well, instead of John and Partner or John and Guest.

If you feel you are going to offend someone, when you send them the invite or hand it to them, just explain that we are limited on numbers and had make tough decisions on who we could invite, this way, that won't feel like you don't like their partner. 

I have been to plenty of weddings, on my own, due to this reason and it hasn't offended me at all. I feel most people, wedding guests understand. 

Remember you won't please everyone and it's your wedding day! 

Guests are fairly understanding about numbers and who is invited. It's really up to you whether you want all guests to bring a plus one, or if you only allocate plus ones to some people. 

An easy way to do this is to address the envelope to the people invited. For example, an envelope addressed to 'Mr and Mrs Smith' only invites the two of them, while an envelope addressed to 'The Smith Family' invites Mr and Mrs Smith along with their three children. An envelope addressed to 'Miss Mariah Jones' only invites her, and an envelope addressed to 'Miss Mariah Jones' and guest' invites her and her guest. If a guest has a specific partner that you're inviting, go ahead and write their names on the envelope. If you'd like to be extra clear about the plus one, you can mention on an information card that only named guests are invited.