As I understand it, a co-opted non-Councillor can sit on a committee or working group and help to drive policies and carry out actions. However he has no voting rights and should not be involved in any financial discussion or staffing matters. He can sit in on a Parish Council meeting but presumably should only take part in discussions that involve his particular areas of influence. And if the PC goes into a confidential session he should be excluded just like any other member of the public.
Have I got those do's and don'ts broadly correct?