If all of the files are in the same folder, and the tracks are all sequential, and they all have the same Album tag, then MC should automatically tag them as a Multiple Artists album. The trick is to not use the [Artist] tag in your view schemes and instead use [Album Artist (auto)]. On a single artist album, [Album Artist (auto)] will have the same value as that artist. On a multiple artists album, the [Album Artist (auto)] tag will say "(Multiple Artists)". If you want it to say something else, then edit the [Album Artist] tag for those files and then [Album Artist (auto)] will have that value instead.
There's a more detailed example in the wiki if you're still confused. I admit it's not the most intuitive solution in the world, but once you get the hang of it, it's really simple and works really well.