Some councils, though not my own have adopted WhatsApp. Look up Mistley Parish Council WhatsApp policy for example. It's really just about making sure it doesn't spill over into discussion about live or upcoming agenda items as that could be seen as pre-determination but that is the same for any other communication. Also the FOI point is well made, although wouldn't apply to a private WhatsApp group. You also need to make sure all councillors have or want to have WhatsApp or opt out, otherwise it could be discriminatory, in which case there is nothing to stop you setting up a non official WhatsApp group without using council resources?