Good for you for stepping up. Unless your committee is a registered charity, which it doesn't sound likely, your only obligation is to do your best and act in the best interest of the members.
As far as I can tell this committee has no legal responsibility, but do check with the council. If I'm correct, it simply organises joint arrangements for the members and helps to carry out new joint projects.
Your job is to chair the meetings. Set an agenda, ie a list of topics to be discussed, bring them to an agreed conclusion and keep the meetings to time and orderly. Your moral responsibility is to be fair and democratic, to allow other members to join the committee or to be heard and to have no conflict of interest. Ie, when you need to club together to buy a new shared water butt, you don't buy one from your own water butt business.
This is a useful little site
http://www.diycommitteeguide.org/article/chairing-meetings