I have done a bit of research into the use of WhatsApp for interest groups through my role as IT and Media officer in the local U3a. My findings, for what they are worth, is that there is not a lot of benefit to be gained. We have the benefit of a forum which everyone is happy to get involved in which leads to protracted discussions at times and also a PM system which is useful. Another level, I feel, would only dilute what we have and may even create cliques within the club.
Of course, there is nothing to stop members forming their own WhatsApp groups, for all I know they probably already have. However, although it is a very efficient means of communication and passing photographs, etc, it doesn't tick all the boxes that the forum does which is the organ to which members associate membership of the club with. If members have regular companions with whom they fish or skippers they crew for, WhatsApp away but, there may be occasions when an offer of a trip may not be wanted to be shared with the whole group. For a loose group of friends who do not have the security blanket of a club, WhatsApp is perfect.
Horses for courses!
Terry.