When you allow the Holy Spirit within you to be your guide, you can say “no” without explanation.
You would be wise to discuss this with your friends to whom you may have to say “no.” Let them know you trust the Lord to help you make decisions about certain (or all) areas of your life.
Help them to understand that He knows what lies ahead in every decision and you can’t just strike out doing your own thing without getting His OK!
And, when you do discern that you need to say “no,” don’t hesitate. Maybe your “no” will help others rethink their plan. The Spirit could lead them to a better decision or plan.
But don’t ever think you owe anyone an explanation for your “no,” especially if you have openly and honestly explained to them who guides your life and how you make decisions!