Amos is almost spot on - but I would substitute 'organisation' for 'group of people'.
Sometimes a think-tank is brought together for a specific task and could include amongst the experts above, others such as affected persons (if a crime-rate group for example) or lateral thinkers or public personlities etc.

It is true that some organisations specialise (and charge loads) in these advisory areas, but not all think-tanks are organisations.

rgds

James