Is there a graceful way to extend a late invitation?

I'd probably just say what you have here! It definitely sounds like she should be there. "I'm so glad we've gotten so close over the past year. We made our guest list so long ago, and it never really got updated! I know it's a little late, but we'd love to have you there. [hand over invite]"

I would talk to her privately. Tell her that you value how much you have gotten to know her lately and how she helped with your shower. Explain that you made your guest list before you knew her well but would like to invite her now even if it is late. Let her know that you understand if she isn't able to make it, but you would really enjoy having her if she is able to come. I am normally opposed to a b-list but u feel like this is different. I ended up extending a late invite to a co-worker with a similar situation. She has told me many times how honored she was to be invited and how happy she was to come.

I think @Natalie nailed it. There's no graceful way to do it, but I think people usually appreciate honesty. I extended a few late invites and explained that we have enormous families and a small venue, so we thought we would have a family/old friends-only event, but as we got declines back, we really wanted to include all the people that have become dear to us in recent years. Everyone that got a late invite understood and were happy to be invited, even if late.

I think in circumstance like this, if as others have suggested, that you explain the reason for the late invite in person, that she would not be offended. I feel like I may end up doing something similar. I have already invited 3 coworkers from my original guest list, but now one of them is working somewhere else but we keep in touch, one I hardly speak to anymore (will still invite, but wouldn't be surprised if he does not show), and the other I am still close with. I have started to get closer to some ladies at work that I didn't know at the time I made my guest list. I feel bad because they are always asking about my wedding and how planning is going and all that. I may extend an invite to them if/when some family members decline. I know this is not generally encouraged, but I think there are some circumstances where as long as you are up front about how you came to invite them late, they would understand and no one would be offended.

I actually did this. Over the school year (I'm a teacher), I got close to a coworker who I coached with. My guest list has been made for awhile, but I just sent out invites. So I let her know I added her and her husband because she has been a great help to me this year, and I feel like she is a really good friend. She was excited, but then rushed to say I didn't have to feel obligated to invite them. I said no obligation, I genuinely would love to have them there to celebrate with us.