We have had the same issues in HK for our main "TMC Whatsapp group". I exported the entire chat to email, pulled it into a spreadsheet and sorted it. This showed who weren't active, or hadn't posted for a while. Those users were contacted directly and asked if they really wanted to stay in the group.
Our Whatsapp group was intended as a way to find each other at the July 23rd, 2014 initial Tesla Model S delivery event in Hong Kong. We kept the group and it grew ... and I have the history since back then, very interesting!
Emailing the chat, btw, is a great way to look for previous discussions. Just open it in a text browser, and use a simple search function to find answers to previous discussed items.