I think Menma can move objects even when in the presence of other people. She made those pastries when Jinta was out of the house, so it can't be that Jinta made them himself in his insanity, unless the show is really trying to fool viewers by twisting even the reality that is displayed onscreen.

I think there is a condition for her to manifest. It might just be that she cannot do it as efficiently if those present reject her existence.