The invite list has a tendency to stir up a good dose of tension. It boils down to dollars almost every time, and it can easily veer into a whole bag of other issues so let's get into trying to navigate it. I'm right there with you in this process so by no means are my responses rules of any kind. Let's call them philosophies.
1. Setting the Number and Ratios
I know what is this math? But rule of thumb, know your venue and your budget before creating the list. From there, typically, the couple gets half the guest list and each set of parents gets 25% of the guest list. This of course gets complicated very easily when say, your parents are divorced and no longer share mutual friends, one family is much larger, or traveling from out of town, or the venue and budget create a hard stop on numbers. Nonetheless, the 50-25-25 split is a good jumping off point for fairness.
2. Guesstimates and What TO DO WHEN THE numbers are higher than you planned
At least 10% will have regrets right? Well the experts actually say that you should save yourself the stress and expect 100% attendance. Then there is no semi-tainted situation where you're opening invites from loved ones but praying they have a conflict. If you do gamble for say 10-15% of your guests to have regrets, and your budget hinges on it, flowers and booze are the easiest ways to cut down.
3. Setting some standards on Why You're Inviting Who You're Inviting
I created a 3-step invite criteria for my invites: 1. Because they mean the world to me, 2. Because I adore them, and 3. It would be unthinkable to get married without them there. Ideally they also know me and my fiance, a classic rule, but one I also find old fashioned. When you've lived in multiple cities, and find the right man at the old age of 30, there are more exceptions than not to who knows you both well.
4. Making Cuts
It feels ruthless to make cuts on something so celebratory, but unless you have endless funds, it has to be done. My personal rule of thumb, which has so far worked really well as a guidepost, is that if I didn't think to tell that friend, personally (ie. not via a social media) that I was engaged within the first week or two of being engaged, then as much as I love them, they don't need to be on my core list. I like this rule because it shows me who the essential people are for me to celebrate the highest of life's highs with.
5. When the parents are outta control
I feel like someone's bound to get out of control with their numbers. Have a glass of wine and talk it out. Make a "reserve list" of people they can invite if some people on their list have regrets. Did I mention have some wine?
6. For The guests who invited you to their wedding but you're not sure they can make your list
In my book, weddings are a place for celebration, not obligation. It's important to remember that budget and venue capacity vary so wildly from wedding to wedding, both of which dictate the invite list, that comparing your guest list to someone else's guest list is truly comparing apples to oranges. Go back up to rule #2 and then decide.
7. When you invite someone in conversation, without factoring in if you can actually fit them into your numbers
I've done this and I've had this done to me; it's called putting your foot in your mouth. If you do truly want them there, hope that you can fit them in once you receive some regrets. If you cannot fit them in, assess the terms of the regrettable verbal invite... Was it when you were both tipsy? Was it very off hand and casual? If so, it's possible to sweep it under the rug, but if it was in a sincere and sober setting, it's probably best to own it and deal with it head on by saying something genuine like we so wanted you there but we were so much more limited in our invite list than I realized. And also, I'm an idiot.
8. inviting people who are not invited to the wedding to an engagement party
This is a divisive one, but in general, I say if you want to celebrate them and you make it clear this is your way of celebrating and including all of the people you love, including those whom you can't invite to the wedding due to [insert reason: budget, venue, etc]. But tread carefully here, and I don't recommend doing this unless there are a handful of these types of people on the engagement party invite list. I also think it's wise to tell your friend in person and in advance about the party's purpose, before sending the engagement party's formal invite so there's no confusion.
9. When a guest without a plus one asks if she/he can bring a plus one
Toughie. I've even seen a guest show up with the plus one without ever communicating that to the bride. I've also been solo at a wedding and that isn't so fun. Assuming your numbers are tight, I don't really buy the theory of "it has to be serious relationship" or "if I know the person." In the end, it's not about if it's a valid relationship or a good person, it's about the numbers. Don't feel you have to answer on the spot; give yourself wiggle room to let them know when more Rsvps come in.
Unless they are among your best friends, this is a fast way to have numbers climb.