If the pieces of cake must have the same size and shape, then I get the feeling this isn't one of those questions of the "one person cuts and another chooses" variety. It's asking how a certain number of slices can result in equally shaped pieces if the guests can cut with absolute precision.
This is a variation on the problem of 2 people trying to split something. [where A cuts and B chooses which half].
One solution that solves this problem, but can be used for any practical value of N people