Whatsapp is encrypted end to end so the security element is pretty tight, but like anything technological could potentially be hijacked. However it is safer than email because emails are not generally encrypted.
Like anything TRANSPARENCY is the key.
Why do you have the whatsapp Group? What’s its core purpose. If the group is to generally discuss business but not make decisions then generally it’s OK, however for transparency the discussions should be saved.
If the group has been set up to create a group of like minded individuals to discuss business and then surprise the content on to others at the full council may cause some hostility.
You also need to be mindful that you’re not discriminating against anyone who is unable to use tech for whatever reason, there must be provision for those people to be involved to.
Additionally you need to be aware of GDPR. Cllr’s or employees may have only agreed to a particular number being made public I.e their landline. By creating a group everyone else can see the numbers of those in the group so do be aware.