I was prepping a new design, and decided to throw this tip out.  When you have an open surface such as the one in this image, and want to add a face to close if off, you can add a surface by using ‘Patch’.

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Ribbon-> Home Tab->Surface Panel->Patch

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It’s quite simple and uses the edges selected to create a face.  One word of warning: Inventor will try to use all the edges selected and create a smoothed face between them, so be careful what you select, and review he results carefully.