I jumped up from Inventor 6 straight to the 9, but, according to its new features list, this must be same in release 8. In the part environment, use the Stitch Surace tool. For a μ-tutorial, I just did the following:
1. In a new part's sketch, make a 15mm square
2. Delete one side
3. Extrude the profile up 25mm
4. Make a sketch on one of the 2 side surfaces and draw a 10mm circle approximately in the center
5. Extrude the cylinder 50mm as a surface, to the both sides of the sketch, so that it intersects another side surface of the "square arch"
6. Use the Split tool and the arch as a knife to split the cylinder into 3 surfaces
7. Use the same Split tool but this time the middle portion of the cyliner as the knife to cut out circles in arch sides
8. Using the Delete face tool, delete cylinder sides that are outside the arch, and circles on the arch sides.
9. Use the Stitch Surface tool to stitch both object. Voilą - a Stitch Surface object appears in the browser.
Just wondering what type of design are you doing? I am asking because I never needed to stitch surfaces, and wondered what it can be good for.
The reason for combining surfaces is that a machining fixture was designed in inventor. When I import that fixture into my CAM software, It imports as 21 different surfaces. My Cam software only allows me to choose one surface as a clamping surface.
Go to the help icon and it will explain the stitch command. There are some limits to what can be stitched (edges of the same size etc.) but the help section will talk you thru it. If it would help and the file is not too large I would be happy to "stitch it up" for you! But you would not learn anything.
Another suggestion, export the drawing (IGES out) and stitch the surfaces after the drawing has been converted. I find with some IGES and STEP files that some of the surfaces are easy (er) to work with.