As a community group, they will have insurance - something akin to a village hall policy. These policies include public liability insurance, which provides insurance.
The people that run the group (possibly as trustees) have a liability to make the premises reasonably safe, but in the event that something did go wrong, the risk will be covered by the insurance.
The building and its land should have a simple risk assessment done perilously and the risk of injury from the pond should appear on that. They could agree to erect a sign to help mitigate the risk and/or fence the pond off.
So, putting the sign up doesn't remove the responsibility; not putting a sign up could be considered not taking reasonable measures to minimise the risk but the insurers would still pay out in the event of a claim.
This is a civil law principle that applies across the UK, the county is irrelevant.