Hi
If you are guiding groups of under 18 over 600 metres in altitude or more than 30 mins walk fom a road you must be registered with the AALS. Otherwise there are no regulations.
That said, a Mountain Bike Leader qualification from a reputable provider, would give you additional knowledge / coaching skills etc. that are required for leading / coaching groups which should therefore greatly minimise the chance of neglegence, as you have of course a duty of care for your group or individual.
The following link may help –
http://www.mountainbikeinstruction.co.uk/cswhy.asp?session=43800101029031231939671220721
Ray Mazey