When you sign up to any service, they ask for an email address. They don't ask for a mobile number necessarily, and there is never a "my mobile number is on WhatsApp" checkbox. Why is the assumption of the organiser that you're on WhatsApp your concern? They have assumed you're on a certain platform, and it's their mistake.
It reminds me of the tidal wave of people suddenly abandoning their own websites and instead using "Find Us On Facebook". They might as well put "Use this keyword on AOL".
Facebook is not the internet, and WhatsApp is not the only communication method.