I think you are asking whether the chairman of a local council can represent the council at a meeting of some other group without the council's authority? The answer to that is no. Apart from a very few matters such as the ability to call council meetings, the chairman does not have any more powers than any other councillor. And no councillor has any authority to act on their own unless specifically authorised by the council. A council can only make decisions when it meets as a council, following the usual rules about notice, etc.