I believe its free on Android for the first year or so. Besides, this app hasn't really updated itself in a long time. groupme is much better. but if your friends are already using whatsapp, then it will be easier to use that. but like other people have said 99 cents isn't bad.
WhatsApp has not been updated for the iPhone 5 as of yet (which is really disappointing) but it is a great way to keep in contact with people from other platforms. I prefer texting but whatsapp is also pretty cool for .99 cents.