Thats fine, I've just not come across a situation where an iPhone user has not moved over to Whatsapp in my large group of friends and family. Statistics never tell the full story. The bottom line is if you want to be able to group message across platforms you have to use a third party solution. iMessage is great if everyone you know owns a iPhone, but if they don't it just does not make sense. Personally i wish Android and iOS would create a cross platform messaging solution, the fragmentation is just silly.
In the case of mixed friends people use Facebook messenger not whatsapp for the most part.