How late is too late to ask someone to stand up in your wedding?

So I have been sitting on the fence about asking 2 girls to stand with me since I got engaged. Both are amazing humans, but I wasn't sure how close they felt to me so I've been waiting and for a while decided to just stick with the 4 people I already asked. Due to an unforeseen issue they both stepped up big time to help me out and now I can't imagine not saying I do with them next to me. My only issue now is that they are friends with my other bridesmaids and know I hadn't planned to ask anyone else. I don't want them to feel like they are an after thought to the other girls because they aren't. How should I go about asking them? I have the proposal boxes at the ready. I'm just waiting for the last few things to come in and to come up with the right words to ask them.