I'm quite new to parish councils but I think this question is conflating two things - the council and individual councillors. Individual councillors can meet with who they like, discuss what like and decide what they like - just as any member of the public can. But none of that would have any effect on the council. The council holds meetings and makes collective decisions in public.
It has to be the case that groups of councillors can meet outside of a council meeting together and with others to agree an approach and strategy. Our neighbouring town council has political groups (con, green, lib dem) and they definitely meet as groups to decide how they will approach the business of the council. They are not actually conducting the business of the council at such meetings, of course, just deciding on an approach - that is how political groups and parties work.
(edit: they must of course not share/discuss any council confidential information with others)