I second Adrir's comments. Particularly the point about including videos of your work. Sure, source code and executables are good, in case the person looking at your protfolio has time and is interested to see how your game worked, but the easiest thing for them is to record a video of what you want them to see, in a common video format.
Accessibility is they key here. If you do include executable, make sure you've tested them on several machines. The last thing you want is for them to encounter an error because they have a different version of DirectX to you.
Also including group projects is not only acceptable, but can be quite a bonus - particularly if you were not the only programmer. When you get a job you will probably have to work in a team, so they should be confident that you can handle that.
If you want projects to work on that you can put into your portfolio, head over to the programming board and look at the programming challenge.