My golden rule. Never do any thing you do not want to do. If they are truly your friends they will respect your rights. Just say, thanks, but I got some thing else going on. No explanations needed. If they persist for a reason, tell them with a smile you really do appreciate the offer but you can't do it this time, catch you next time! After that just keep smiling and say, "sorry"

if that doesn't work there is always Plan B.

Start talking religious