You can use either because WhatsApp and the internet in general are new enough that we haven't yet agreed on a formal rule for them. In general, we use ON for larger and more public things and IN for smaller and more private things. For example, we ride ON the bus, but we ride IN a car. By that logic, we would post ON WhatsApp (a big public forum) but IN a WhatsApp group (a small private forum). I suspect that is what will someday become the rule.

You can say either, but you need to remember to use articles (like a/an/the) properly. Moreover, if you want to For example, you say either of the following sentences: (1.) I posted a message in the WhatsApp group. (2.) I posted a message on WhatsApp. (3.) I sent a message to the members of our Whatsapp group.

IN would be the most natural-sounding option :)

"ON"