Rose76 hit on the best advice I ever got- which is don't make a lot of excuses or justify yourself. Just directly state your position in a polite way. A good response to the party might be something like "That sounds like it will be a lot of fun, but I'm afraid I won't be able to make it." If they press, then you just state a simple point and dont expand on it (that gives your power away) something like "I'm busy that weekend."

At one point I thought I was good at boundaries, and while I was to some extent, I wasn't actually comfortable doing it. The key is to feel comfortable and confident in your choices, then you aren't inclined to make up excuses or over explain yourself. It's your life, you get to decide what you want to do and not do!

I despise group text/chats for the most part because they are very disruptive IME. DH's family has a group chat (of which I am not a part both by choice and not). He has such FOMO that he will ignore the people he's with, or activities he's engaged in, to make sure he doesn't fall out of favor with the group