For this kind of thing, I’d suggest you use the Post Object field instead. It’s easier and you can get the staff data if you need it later.
If you still want to use checkboxes, please take a look at this page: http://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/.
Instead of using get_field() to get the choices, you need to use the get_posts() function, loop over the returned posts and set the choices based on this data.
I hope this helps.
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