Repeating words in enumeration: '... senior executives, decision makers and policy makers.'

Both ways are grammatically correct, and this seems to be a matter of style. It seems that it's important to consider whether it's written text (e-mail, ad prospect) or a speech in front of the audience.

In the latter case you might want to try both variants and achieve different effects. Saying 'decision and policy makers' with large emphasis on 'and' might break the monotony. But if you want to achieve the monotony and strict symmetry the original phrasing is good.

Your original phrasing

senior executives, decision makers and policy makers

is correct. If it's desired to state "makers" only once, then "decision and policy" is a list of two items which needs an "and" before its last item; thus "decision and policy makers". This in turn is the second item in a list of two; this list needs an "and" before its last item. So we get

senior executives and decision and policy makers

Both "ands" are needed. If you'd written

senior executives, decision and policy makers

then the reader would have parsed the "and" as part of the item "decision and policy makers". This results in two items with just a comma between them and no "and". Because there is no "and", the reader would thus expect at least one further item (with an "and" before the last item).