There are a number of ways you could approach this. Media Center supports "List" type fields. A "List" type field can store multiple values, separated by a semicolon. So, if the Artist field contained "Dave Matthews;Jerry Garcia", MC would show the song under both of those artists in the view schemes and smartlists. It would NOT show the song under the single artist, "Dave Matthews;Jerry Garcia".
One problem with this approach is that MC doesn't let you change the type of default fields, such as Artist. So you'd need to define a custom field and use that field for multi-artist tracks. For example, you could use the standard Artist field for the "primary" artist and define a second field, maybe named, "Multiple Artists" or "All Artists". That field could be "List" type.
Another reason that you don't want to change the standard "Artist" field is that the "Artist" field for most formats (MP3, WMA) is standardized. That means that if you tried to use it to store list-type data, most other (non-MC) programs would get confused.
Personally, for multi-artist tracks, I just store the primary artist in the Artist field and leave it at that. I put "Easy Lover" under Phil Collins even though there's some other famous guy on that track too. Michael Stipe sings on the Indigo Girls song, "Kid Fears", but when I'm looking for that song, I think, "Indigo Girls". But I just don't have that many multi-artist tracks, so it's not much of an issue for me.