“If you aren’t able to commit to a full body yes, it is a lot more respectful to say thank you, you’re so kind, I’ll consider the invitation, please give me time, so that you when you do finally say yes, it is a full body yes”.
When you agree to join someone in a thing they have invited you to or take on an extra assignment at work when already stretched, consider the domino effect for yourself and for the other person.